about:config is Firefox's advanced settings panel — hundreds of knobs most users never touch. Here's which ones are actually useful for developers (not the paranoia list, not the full list — the practical ones).
Accessing about:config
Type about:config in the address bar. Accept the warning. Search for settings by name.
Settings are:
- Boolean: true/false toggle (double-click to toggle)
- Integer: number (double-click to edit)
- String: text value (double-click to edit)
Reset to default: right-click any modified setting → Reset.
Development Convenience
// Show full URL in address bar (no truncation)
browser.urlbar.trimURLs = false
// Disable Accessibility Service (can improve DevTools performance on Windows)
accessibility.force_disabled = 1
// Allow invalid SSL certificates on localhost (useful for local HTTPS dev)
// Note: changes security.tls.insecure_fallback_hosts or use about:certificate-error workaround
// More commonly: visit about:certificate-error on the localhost page, click Advanced > Accept Risk
// Enable CSS Grid and Flexbox inspector badges in DevTools
// These are now on by default in recent Firefox, but if not:
devtools.inspector.flexboxHighlighter.enabled = true
Performance Tweaks
// Number of HTTP connections per server (default: 6, max 15)
network.http.max-persistent-connections-per-server = 10
// DNS cache size (default: 400 entries)
network.dnsCacheEntries = 1000
// Disk cache size in KB (default: 256000 = 256MB)
browser.cache.disk.capacity = 512000
// Enable hardware acceleration (if disabled for troubleshooting)
gfx.direct2d.disabled = false (Windows)
layers.acceleration.force-enabled = true (verify your GPU supports this)
Privacy / Security
// Force prefers-color-scheme: dark (dark mode for all sites)
ui.systemUsesDarkTheme = 1
// Disable WebRTC IP leak (important if using VPN)
media.peerconnection.ice.default_address_only = true
// Disable Pocket integration
extensions.pocket.enabled = false
// Disable telemetry
toolkit.telemetry.unified = false
toolkit.telemetry.enabled = false
datareporting.healthreport.uploadEnabled = false
// Disable Firefox Studies (Shield)
app.shield.optoutstudies.enabled = false
Tab and Session Behavior
// Load previous session tabs lazily (only load when clicked)
browser.sessionstore.restore_on_demand = true
// How often Firefox saves session data (default: 15000ms = 15s)
browser.sessionstore.interval = 15000
// Lower for more frequent saves, higher for less disk I/O
// Number of closed tabs Firefox remembers for Ctrl+Shift+T
browser.sessionstore.max_tabs_undo = 25 (default: 10)
// Number of closed windows Firefox remembers
browser.sessionstore.max_windows_undo = 5
Display / UI
// Default font size (default: 16, you can set to anything)
font.size.variable.x-western = 16
// Minimum font size (prevents sites from using tiny text)
font.minimum-size.x-western = 0 (set to 10 or 12 to enforce minimum)
// Scroll speed multiplier
mousewheel.default.delta_multiplier_y = 100 (increase to scroll faster)
// Smooth scrolling
general.smoothScroll = true (already default true)
Network / Fetch Behavior
// Speculative pre-connections (Firefox pre-connects to hovered links)
network.http.speculative-parallel-limit = 6 (0 to disable)
// Prefetch linked pages
network.prefetch-next = true (false to disable)
// Send referer header only to same-domain origins
network.http.referer.XOriginPolicy = 2 (stricter privacy)
// Upgrade insecure requests automatically
dom.security.https_only_mode = true (same as HTTPS-only mode UI toggle)
about:config for Web Developers Specifically
// Enable experimental CSS features (varies by Firefox version)
// Check about:config for "layout.css.experimental" to see what's available
// Enable Service Worker debugging in private windows
dom.serviceWorkers.testing.enabled = false (true to enable testing)
// Allow CORS preflight cache (useful for API debugging)
// Not typically an about:config setting — handle at server level
// Enable Web Sockets in DevTools network panel
// Usually enabled by default; check devtools.netmonitor.features.webSockets
How to Find Settings
Searching in about:config is good for known settings. To discover new ones:
The arkenfox user.js project documents hundreds of settings with explanations — good reference even if you don't use it as a config file.
Mozilla's wiki has
category:Firefox_configurationpages.When debugging a specific behavior, add your suspicion to about:config search.
user.js: Persisting Settings
Changes in about:config are lost if Firefox resets its profile. For settings you want to persist across Firefox updates:
Create user.js in your profile folder (same location as prefs.js) with:
user_pref("browser.urlbar.trimURLs", false);
user_pref("browser.sessionstore.max_tabs_undo", 25);
user_pref("media.peerconnection.ice.default_address_only", true);
Firefox reads this file at startup and applies the settings. The file overrides prefs.js, so your preferences survive profile updates.
The most impactful ones for most developers: browser.sessionstore.restore_on_demand (reduces RAM from session restores), network.http.max-persistent-connections-per-server (slightly faster multi-resource pages), and the privacy settings above.
