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Firefox about:config: The Settings That Actually Matter for Developers

#firefox #javascript #webdev #productivity

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
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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)
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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
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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
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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)
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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)
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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
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How to Find Settings

Searching in about:config is good for known settings. To discover new ones:

  1. The arkenfox user.js project documents hundreds of settings with explanations — good reference even if you don't use it as a config file.

  2. Mozilla's wiki has category:Firefox_configuration pages.

  3. 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);
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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.

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