diff --git a/app/api/auth/session/route.ts b/app/api/auth/session/route.ts
index 0e054004..a21f526f 100644
--- a/app/api/auth/session/route.ts
+++ b/app/api/auth/session/route.ts
@@ -13,6 +13,7 @@ import { configManager } from '@/lib/admin/config-manager';
 import { isPublicHttpUrl } from '@/lib/security/url-guard';
 import { recordLogin } from '@/lib/telemetry/login-tracker';
 import { parseJmapServers, resolveTrustedJmapUrl } from '@/lib/admin/jmap-servers';
+import { MAX_ACCOUNT_SLOTS } from '@/lib/account-utils';
 
 const COOKIE_OPTIONS = {
   ...getCookieOptions(),
@@ -23,7 +24,7 @@ function getSlot(request: NextRequest): number {
   const raw = request.nextUrl.searchParams.get('slot');
   if (raw === null) return 0;
   const slot = parseInt(raw, 10);
-  if (isNaN(slot) || slot < 0 || slot > 4) return 0;
+  if (isNaN(slot) || slot < 0 || slot >= MAX_ACCOUNT_SLOTS) return 0;
   return slot;
 }
 
@@ -70,7 +71,7 @@ export async function POST(request: NextRequest) {
       return NextResponse.json({ error: 'JMAP server not configured' }, { status: 500 });
     }
 
-    const slot = typeof bodySlot === 'number' && bodySlot >= 0 && bodySlot <= 4 ? bodySlot : getSlot(request);
+    const slot = typeof bodySlot === 'number' && bodySlot >= 0 && bodySlot < MAX_ACCOUNT_SLOTS ? bodySlot : getSlot(request);
     const cookieName = sessionCookieName(slot);
     const authHeader = `Basic ${Buffer.from(`${username}:${password}`).toString('base64')}`;
     const normalizedServerUrl = await verifyJmapAuth(upstreamUrl, authHeader, { trusted: upstreamTrusted });
@@ -185,8 +186,8 @@ export async function DELETE(request: NextRequest) {
     const all = request.nextUrl.searchParams.get('all') === 'true';
 
     if (all) {
-      // Delete all session cookies (slots 0-4)
-      for (let i = 0; i <= 4; i++) {
+      // Delete all session cookies across every slot.
+      for (let i = 0; i < MAX_ACCOUNT_SLOTS; i++) {
         cookieStore.delete(sessionCookieName(i));
         clearStalwartAuthContextInStore(cookieStore, i);
       }
diff --git a/app/api/auth/stalwart-context/route.ts b/app/api/auth/stalwart-context/route.ts
index 2edb1630..7894a255 100644
--- a/app/api/auth/stalwart-context/route.ts
+++ b/app/api/auth/stalwart-context/route.ts
@@ -6,9 +6,10 @@ import { configManager } from '@/lib/admin/config-manager';
 import { isPublicHttpUrl } from '@/lib/security/url-guard';
 import { recordLogin } from '@/lib/telemetry/login-tracker';
 import { parseJmapServers, resolveTrustedJmapUrl } from '@/lib/admin/jmap-servers';
+import { MAX_ACCOUNT_SLOTS } from '@/lib/account-utils';
 
 function getSlot(request: NextRequest, bodySlot: unknown): number {
-  if (typeof bodySlot === 'number' && bodySlot >= 0 && bodySlot <= 4) {
+  if (typeof bodySlot === 'number' && bodySlot >= 0 && bodySlot < MAX_ACCOUNT_SLOTS) {
     return bodySlot;
   }
 
@@ -16,7 +17,7 @@ function getSlot(request: NextRequest, bodySlot: unknown): number {
   if (raw === null) return 0;
 
   const slot = parseInt(raw, 10);
-  return Number.isNaN(slot) || slot < 0 || slot > 4 ? 0 : slot;
+  return Number.isNaN(slot) || slot < 0 || slot >= MAX_ACCOUNT_SLOTS ? 0 : slot;
 }
 
 export async function POST(request: NextRequest) {
diff --git a/app/api/auth/token/route.ts b/app/api/auth/token/route.ts
index 1fa3377b..e734ba71 100644
--- a/app/api/auth/token/route.ts
+++ b/app/api/auth/token/route.ts
@@ -4,12 +4,13 @@ import { logger } from '@/lib/logger';
 import { refreshTokenCookieName, refreshTokenServerCookieName } from '@/lib/oauth/tokens';
 import { exchangeCodeForTokens, buildOAuthParams, getMetadata, getTokenEndpoint } from '@/lib/oauth/token-exchange';
 import { getCookieOptions } from '@/lib/oauth/cookie-config';
+import { MAX_ACCOUNT_SLOTS } from '@/lib/account-utils';
 
 function getSlot(request: NextRequest): number {
   const raw = request.nextUrl.searchParams.get('slot');
   if (raw === null) return 0;
   const slot = parseInt(raw, 10);
-  if (isNaN(slot) || slot < 0 || slot > 4) return 0;
+  if (isNaN(slot) || slot < 0 || slot >= MAX_ACCOUNT_SLOTS) return 0;
   return slot;
 }
 
@@ -21,7 +22,7 @@ export async function POST(request: NextRequest) {
       return NextResponse.json({ error: 'Missing required parameters' }, { status: 400 });
     }
 
-    const slot = typeof bodySlot === 'number' && bodySlot >= 0 && bodySlot <= 4 ? bodySlot : getSlot(request);
+    const slot = typeof bodySlot === 'number' && bodySlot >= 0 && bodySlot < MAX_ACCOUNT_SLOTS ? bodySlot : getSlot(request);
     const serverId = typeof bodyServerId === 'string' && bodyServerId ? bodyServerId : null;
 
     const tokens = await exchangeCodeForTokens(code, code_verifier, redirect_uri, serverId);
@@ -112,9 +113,9 @@ export async function DELETE(request: NextRequest) {
     const all = request.nextUrl.searchParams.get('all') === 'true';
 
     if (all) {
-      // Revoke and delete all refresh token cookies (slots 0-4)
+      // Revoke and delete all refresh token cookies across every slot.
       const cookieStore = await cookies();
-      for (let i = 0; i <= 4; i++) {
+      for (let i = 0; i < MAX_ACCOUNT_SLOTS; i++) {
         const name = refreshTokenCookieName(i);
         const serverCookieName = refreshTokenServerCookieName(i);
         const token = cookieStore.get(name)?.value;
diff --git a/app/api/auth/totp-token-exchange/route.ts b/app/api/auth/totp-token-exchange/route.ts
index 85f2025d..4c120c40 100644
--- a/app/api/auth/totp-token-exchange/route.ts
+++ b/app/api/auth/totp-token-exchange/route.ts
@@ -9,6 +9,7 @@ import { configManager } from '@/lib/admin/config-manager';
 import { isPublicHttpUrl } from '@/lib/security/url-guard';
 import { recordLogin } from '@/lib/telemetry/login-tracker';
 import { parseJmapServers, findServerByUrl, findServerById } from '@/lib/admin/jmap-servers';
+import { MAX_ACCOUNT_SLOTS } from '@/lib/account-utils';
 
 /**
  * Exchange basic auth credentials (with TOTP appended) for OAuth tokens.
@@ -85,7 +86,7 @@ export async function POST(request: NextRequest) {
       return NextResponse.json({ error: 'Missing required parameters' }, { status: 400 });
     }
 
-    const slot = typeof bodySlot === 'number' && bodySlot >= 0 && bodySlot <= 4 ? bodySlot : 0;
+    const slot = typeof bodySlot === 'number' && bodySlot >= 0 && bodySlot < MAX_ACCOUNT_SLOTS ? bodySlot : 0;
     const requestedServerId = typeof bodyServerId === 'string' && bodyServerId ? bodyServerId : null;
 
     // Pin the upstream URL to a configured JMAP server. The list of allowed
diff --git a/app/api/settings/route.ts b/app/api/settings/route.ts
index d03a1780..a6f6da6b 100644
--- a/app/api/settings/route.ts
+++ b/app/api/settings/route.ts
@@ -7,6 +7,7 @@ import { readStalwartAuthContextFromStore } from '@/lib/stalwart/auth-context';
 import { saveUserSettings, loadUserSettings, deleteUserSettings } from '@/lib/settings-sync';
 import { configManager } from '@/lib/admin/config-manager';
 import { readFileEnv } from '@/lib/read-file-env';
+import { MAX_ACCOUNT_SLOTS } from '@/lib/account-utils';
 
 function classifyError(error: unknown): { message: string; status: number } {
   const code = (error as NodeJS.ErrnoException).code;
@@ -59,7 +60,7 @@ function normalizeUrl(url: string): string {
 
 /**
  * Verify identity against session cookies across all account slots.
- * With multi-account, the requesting account may be on any slot (0-4).
+ * With multi-account, the requesting account may be on any slot.
  * Checks both basic-auth session cookies and stalwart auth context cookies
  * (used by OAuth/SSO and TOTP-upgraded sessions).
  * Returns true only if a matching cookie is found.
@@ -68,7 +69,7 @@ async function verifyIdentity(username: string, serverUrl: string): Promise<bool
   const cookieStore = await cookies();
   const normalizedServerUrl = normalizeUrl(serverUrl);
 
-  for (let slot = 0; slot <= 4; slot++) {
+  for (let slot = 0; slot < MAX_ACCOUNT_SLOTS; slot++) {
     // Check basic-auth session cookie
     const token = cookieStore.get(sessionCookieName(slot))?.value;
     if (token) {
diff --git a/components/layout/account-switcher.tsx b/components/layout/account-switcher.tsx
index 6cf3573a..095c4640 100644
--- a/components/layout/account-switcher.tsx
+++ b/components/layout/account-switcher.tsx
@@ -6,7 +6,7 @@ import { Check, Plus, LogOut, Star, ChevronDown, AlertCircle } from "lucide-reac
 import { useTranslations } from "next-intl";
 import { useAccountStore, type AccountEntry } from "@/stores/account-store";
 import { useAuthStore } from "@/stores/auth-store";
-import { getInitials, MAX_ACCOUNTS } from "@/lib/account-utils";
+import { getInitials, getMaxAccounts } from "@/lib/account-utils";
 import { cn } from "@/lib/utils";
 import { useRouter } from "@/i18n/navigation";
 
@@ -220,7 +220,7 @@ export function AccountSwitcher({ variant = "rail", className }: AccountSwitcher
           </div>
 
           {/* Separator + Add Account */}
-          {accounts.length < MAX_ACCOUNTS && (
+          {accounts.length < getMaxAccounts() && (
             <div className="border-t border-border">
               <button
                 onClick={handleAddAccount}
diff --git a/components/layout/navigation-rail.tsx b/components/layout/navigation-rail.tsx
index f9101427..7ff5fb8c 100644
--- a/components/layout/navigation-rail.tsx
+++ b/components/layout/navigation-rail.tsx
@@ -18,7 +18,7 @@ import { useAuthStore } from "@/stores/auth-store";
 import { useAccountStore } from "@/stores/account-store";
 import { useUpdateStore, selectHasUpdate } from "@/stores/update-store";
 import { getActiveAccountSlotHeaders } from "@/lib/auth/active-account-slot";
-import { getInitials, MAX_ACCOUNTS } from "@/lib/account-utils";
+import { getInitials, getMaxAccounts } from "@/lib/account-utils";
 import { cn, formatFileSize } from "@/lib/utils";
 import { PluginSlot } from "@/components/plugins/plugin-slot";
 import { KeyboardShortcutsModal } from "@/components/keyboard-shortcuts-modal";
@@ -634,7 +634,7 @@ export function NavigationRail({
                 </button>
               );
             })}
-            {accounts.length < MAX_ACCOUNTS && (
+            {accounts.length < getMaxAccounts() && (
               <button
                 onClick={() => router.push(`/login?mode=add-account` as never)}
                 className="flex items-center justify-center w-8 h-8 rounded-full border border-dashed border-muted-foreground/50 text-muted-foreground hover:border-foreground hover:text-foreground hover:bg-muted transition-colors flex-shrink-0"
diff --git a/lib/account-utils.ts b/lib/account-utils.ts
index baaad65c..b9e95cf6 100644
--- a/lib/account-utils.ts
+++ b/lib/account-utils.ts
@@ -62,5 +62,43 @@ export function getAccountScopedKey(baseKey: string, accountId: string): string
   return `${baseKey}::${accountId}`;
 }
 
-/** Maximum number of accounts allowed */
-export const MAX_ACCOUNTS = 5;
+/**
+ * Hard upper bound on cookie slots. Each slot can hold up to ~3 cookies
+ * (session, refresh token, server id, auth context), so 50 slots ≈ 125
+ * cookies on average — within Firefox's per-domain limit of 150.
+ */
+export const MAX_ACCOUNT_SLOTS = 50;
+
+/**
+ * UX cap for browsers using HTTP/1.1. Each account holds one persistent
+ * SSE connection for JMAP push; HTTP/1.1 caps origins at 6 concurrent
+ * connections, so 5 accounts leave one connection free for normal traffic.
+ * On HTTP/2+ this cap doesn't apply because streams are multiplexed.
+ */
+export const MAX_ACCOUNTS_HTTP1 = 5;
+
+/**
+ * Detect whether the page has observed any HTTP/2 or HTTP/3 traffic.
+ *
+ * We walk recent resource-timing entries and treat a single h2/h3 sighting
+ * as a positive signal. Cross-origin entries may report an empty
+ * `nextHopProtocol` without `Timing-Allow-Origin`, in which case we
+ * under-detect and fall back to the conservative cap — that's safe.
+ */
+export function isHttp2Available(): boolean {
+  if (typeof performance === 'undefined') return false;
+  const entries = performance.getEntriesByType('resource') as PerformanceResourceTiming[];
+  for (let i = entries.length - 1; i >= 0; i--) {
+    const proto = entries[i].nextHopProtocol;
+    if (proto === 'h2' || proto === 'h3') return true;
+  }
+  return false;
+}
+
+/**
+ * Effective per-browser account cap. Lifts to {@link MAX_ACCOUNT_SLOTS}
+ * once HTTP/2+ is observed, otherwise returns {@link MAX_ACCOUNTS_HTTP1}.
+ */
+export function getMaxAccounts(): number {
+  return isHttp2Available() ? MAX_ACCOUNT_SLOTS : MAX_ACCOUNTS_HTTP1;
+}
diff --git a/lib/auth/session-cookie.ts b/lib/auth/session-cookie.ts
index f91ac781..685c9a5d 100644
--- a/lib/auth/session-cookie.ts
+++ b/lib/auth/session-cookie.ts
@@ -1,7 +1,7 @@
 export const SESSION_COOKIE = 'jmap_session';
 export const SESSION_COOKIE_MAX_AGE = 30 * 24 * 60 * 60;
 
-/** Get the cookie name for a given account slot (0-4). Slot 0 uses the legacy name. */
+/** Get the cookie name for a given account slot. Slot 0 uses the legacy name. */
 export function sessionCookieName(slot: number): string {
   return slot === 0 ? SESSION_COOKIE : `${SESSION_COOKIE}_${slot}`;
 }
diff --git a/lib/oauth/tokens.ts b/lib/oauth/tokens.ts
index ce69eaa8..57e9b949 100644
--- a/lib/oauth/tokens.ts
+++ b/lib/oauth/tokens.ts
@@ -4,7 +4,7 @@ export const OAUTH_SCOPES = process.env.OAUTH_SCOPES || (EXTRA_SCOPES ? `${DEFAU
 export const REFRESH_TOKEN_COOKIE = 'jmap_rt';
 export const REFRESH_TOKEN_SERVER_COOKIE = 'jmap_rts';
 
-/** Get the cookie name for a given account slot (0-4). Slot 0 uses the legacy name. */
+/** Get the cookie name for a given account slot. Slot 0 uses the legacy name. */
 export function refreshTokenCookieName(slot: number): string {
   return slot === 0 ? REFRESH_TOKEN_COOKIE : `${REFRESH_TOKEN_COOKIE}_${slot}`;
 }
diff --git a/lib/stalwart/credentials.ts b/lib/stalwart/credentials.ts
index c84cd2c0..0c2cd566 100644
--- a/lib/stalwart/credentials.ts
+++ b/lib/stalwart/credentials.ts
@@ -2,6 +2,7 @@ import { cookies } from 'next/headers';
 import { NextRequest } from 'next/server';
 import { sessionCookieName } from '@/lib/auth/session-cookie';
 import { readStalwartAuthContextFromStore } from '@/lib/stalwart/auth-context';
+import { MAX_ACCOUNT_SLOTS } from '@/lib/account-utils';
 
 export interface StalwartCredentials {
   /** URL of the JMAP server (used for JMAP + management method calls) */
@@ -15,14 +16,16 @@ export interface StalwartCredentials {
 function parseSlot(raw: string | null): number | null {
   if (raw === null) return null;
   const slot = parseInt(raw, 10);
-  return Number.isNaN(slot) || slot < 0 || slot > 4 ? null : slot;
+  return Number.isNaN(slot) || slot < 0 || slot >= MAX_ACCOUNT_SLOTS ? null : slot;
 }
 
+const ALL_SLOTS = Array.from({ length: MAX_ACCOUNT_SLOTS }, (_, i) => i);
+
 function getCandidateSlots(request: NextRequest): number[] {
   const requestedSlot = parseSlot(request.headers.get('X-JMAP-Cookie-Slot'))
     ?? parseSlot(request.nextUrl.searchParams.get('slot'));
 
-  return requestedSlot === null ? [0, 1, 2, 3, 4] : [requestedSlot];
+  return requestedSlot === null ? ALL_SLOTS : [requestedSlot];
 }
 
 export async function getStalwartCredentials(request: NextRequest): Promise<StalwartCredentials | null> {
diff --git a/stores/account-store.ts b/stores/account-store.ts
index 2bb8759e..93f452ac 100644
--- a/stores/account-store.ts
+++ b/stores/account-store.ts
@@ -1,6 +1,6 @@
 import { create } from 'zustand';
 import { persist } from 'zustand/middleware';
-import { generateAccountId, generateAvatarColor, MAX_ACCOUNTS } from '@/lib/account-utils';
+import { generateAccountId, generateAvatarColor, getMaxAccounts } from '@/lib/account-utils';
 
 export interface AccountEntry {
   /** Unique key: `${username}@${serverHostname}` */
@@ -13,7 +13,7 @@ export interface AccountEntry {
   username: string;
   /** Authentication mode */
   authMode: 'basic' | 'oauth';
-  /** Cookie slot index (0–4) for session/token cookies */
+  /** Cookie slot index for session/token cookies (0 ≤ slot < MAX_ACCOUNT_SLOTS) */
   cookieSlot: number;
   /** Whether "Remember Me" was checked (basic auth only) */
   rememberMe: boolean;
@@ -80,8 +80,9 @@ export const useAccountStore = create<AccountState>()(
           return id;
         }
 
-        if (state.accounts.length >= MAX_ACCOUNTS) {
-          throw new Error(`Maximum of ${MAX_ACCOUNTS} accounts reached`);
+        const max = getMaxAccounts();
+        if (state.accounts.length >= max) {
+          throw new Error(`Maximum of ${max} accounts reached`);
         }
 
         const cookieSlot = state.getNextCookieSlot();
@@ -178,10 +179,9 @@ export const useAccountStore = create<AccountState>()(
 
       getNextCookieSlot: () => {
         const used = new Set(get().accounts.map((a) => a.cookieSlot));
-        for (let i = 0; i < MAX_ACCOUNTS; i++) {
-          if (!used.has(i)) return i;
-        }
-        return 0; // fallback, shouldn't happen if max is enforced
+        let i = 0;
+        while (used.has(i)) i++;
+        return i;
       },
 
       hasAccount: (username, serverUrl) => {