Firefox about:config exposes internal preferences that do not all appear in the normal Settings window. It is powerful enough to deserve caution, but a few well-known preferences are easy to change, easy to reset, and useful for everyday browsing.
Does this affect you?
Use this for Firefox on Windows, Mac, or Linux if you want safe about:config changes without blindly applying a huge tweak list. Every setting below can be reset individually, so the changes are not permanent.
Open about:config and change a preference safely
The process is the same for most preferences.
- Click the address bar, type about:config, and press Enter.
- Accept the warning. It is normal and appears because advanced settings can affect Firefox behavior.
- Search for the exact preference name.
- For true or false values, click the toggle icon at the right of the row.
- For numbers or text, click the edit icon, enter the new value, and confirm with the checkmark.
A few safe tweaks to try
These are low-risk and simple to undo.
- browser.tabs.warnOnClose: Set it to true if you want Firefox to warn before closing a window with multiple tabs.
- browser.sessionstore.interval: Controls how often Firefox saves open tabs for crash recovery, in milliseconds. Lowering 15000 to 5000 saves more often with a small performance tradeoff.
- media.autoplay.default: Use 1 to block autoplay with sound, or 5 to block all autoplay including muted video.
- browser.urlbar.suggest.searches: Set it to false if you want the address bar to stop showing search-engine suggestions while keeping history and bookmark suggestions.
More control
Reset individual changes
Changed preferences appear in bold and show a reset icon. Click the reset icon to restore that single preference to Firefox default without remembering the previous value.
Rebuild preferences only if needed
If you made many changes and want a clean slate, open about:support, find the profile folder, close Firefox, and rename prefs.js. Firefox rebuilds it on next launch. Use this after trying individual resets.
Avoid old mass-tweak lists
Lists with dozens of settings often include removed preferences, settings meant for different use cases, or changes with tiny gains and real tradeoffs. Change only settings whose purpose you understand.
Sync does not carry every tweak
about:config changes apply to the local Firefox profile. They do not behave like bookmarks or saved logins that move through Firefox Sync.
Sources
- Mozilla Support – About:config editor for Firefox (2025)
- Mozilla – Firefox Source Docs: Preferences system (2025)