Use about: to display information about your Firefox installation, including version, copyright, license, and other useful information.
about:buildconfig shows which options were used to create the current version of Firefox.
about:cache displays both the disk and memory cache, and optionally displays individual entries in each cache.
about:config, Firefox's Configuration Console, enables you to configure many features of Firefox without having to edit prefs.js or user.js. The settings in user.js override these settings, however.
Backing up is the best way to ensure that, if something goes wrong, you can recover. It is better to have too many backups than not enough! Disk space is cheap; your time is not.
In about:config you can add preferences that are not already present. Many preferences that are not in about:config can be added to change Firefox.
Use filtering and sorting to find preferences quickly in about:config.
There are many places to find preferences that are not already in about:config. For instance, the Internet has a wealth of sites that list preferences and their options. Also, a file called all.js lists many of the possible preferences with explanations of valid settings for them. A good starting point is http://preferential.mozdev.org/preferences.html.
To find out who has helped work on Firefox, use about:credits. This lists the approximately 1,000 people who have programmed, tested, and worked on Firefox.
The little-documented about:Mozilla provides a light moment. Try it.
Plug-ins are operating system objects, available to all installed browsers. Most plug-ins do not have to be reinstalled when changing or updating the browser.