From MozillaZine Knowledge Base
This article contains instructions to uninstall Firefox and remove it from your system. You should set another default browser if you are not going to reinstall Firefox and it is currently the default . You can use the information in this article as a general guide.
Important: It is rarely necessary to remove Firefox to repair it. Usually it is sufficient to create a new profile, and it is not necessary to remove the old profile. In some cases it is also necessary to uninstall Firefox and install it in a new folder. Note that it is usually safer to rename files and folders than to delete them.
On Mac OS X
If you also wish to remove your Firefox user data and settings (bookmarks, extensions, etc.) trash the "Firefox" folder, located here: ~Library/Application Support/Firefox.  See Removing user profile data (below) for additional information.
Remove the firefox Installation directory, usually: /usr/lib/firefox-<version>. For example, if using Firefox 1.0 it would be: /usr/lib/firefox-1.0
Exit Firefox completely, open the Windows Control Panel and then,
Note: In Firefox 30.0 and below, the uninstaller includes the option, "Remove my Firefox personal data and customizations". This removes your Firefox user profile data (bookmarks, passwords, cookies, extensions, preferences, etc.). DO NOT select this option if you want to keep your Firefox profile data and settings. This option has been removed in Firefox 31 so, if you wish to remove your Firefox user profile data, you will need to do it manually (see below for instructions). 
Even if the uninstall fails, as it may in some cases, continue on with the rest of the uninstall instructions.
The Firefox uninstall will leave behind some Windows registry entries. Normally, these extra entries are harmless and it is not necessary to remove them. Note: Registry editing is a potentially hazardous undertaking. Also avoid using 3rd party uninstallers (e.g. Revo uninstaller) since these tools can sometimes do more harm than good! 
Removing user profile data
Note: In Firefox 30 and below on Windows, you can use the Firefox uninstaller option, "Remove my Firefox personal data and customizations" (see above) instead of following these instructions.
This step is only recommended if you want to completely remove Firefox from your system, and you do not want to save any of your data. This will remove your Firefox user profile data, which includes bookmarks, passwords, cookies, preference settings and added extensions.
Rename (or delete) the Firefox folder in the default profile path (or move it to a backup location). For example, on Windows 7, press "Windows key + R" to open the Run box
Caution: Make sure that you rename or delete the entire Firefox profile folder, which includes the profiles.ini file that keeps track of profiles. Do not simply rename or delete the profile folder that contains your user data, as doing so can result in a profile cannot be loaded or a "profile in use"/"already running" message when you next attempt to run Firefox (bug 278860).
Related bug reports