How to easily back-up your about:config settings in FireFox?

You can now easily take a back-up of all the settings in the about:config of Firefox. Firefox does show which about:config changes have been set manually, but this isn't always the most accurate way to find out what you changed - So I would always recommend you to take a back-up.

To make a backup of your preferences in Firefox, just make a copy of the file prefs.js, which is kept in your Firefox profile folder. If you mess something up, you can always copy this file back in. (Be sure to shut down Firefox before making a copy of prefs.js or moving a copy back into the profile folder!)

In Windows XP, the profile folder is
\Documents and Settings\\Application Data\Mozilla\Firefox\Profiles\.default\

In Windows Vista, this folder is
\Users\\AppData\Roaming\Mozilla\Firefox\Profiles\.default\

Note that Application Data and AppData are hidden folders by default, so they may not show up unless you force Explorer to show hidden objects. (Open the Control Panel, double-click Folder Options, select the View tab, select "Show hidden files and folders" and click OK.)

In Mac OS X, the profile folder is
/Library/Application Support/Firefox/Profiles/.default/

and in Linux it's
~/.mozilla/firefox/.default/

but on those platforms it's usually quicker simply to search for prefs.js.

Alternatively, you can use the handy Firefox Extension Backup Extension (FEBE). It backs up not only the prefs.js file but just about every other thing in Firefox -- extensions, themes, cookies, form history and so on.