On Windows
Exit Firefox completely, open the Windows Control Panel and then,
On Windows 7/Vista, go to "Programs -> Programs and Features", select Mozilla Firefox <version> and click click Uninstall.
On Windows XP (or below), double-click "Add or Remove Programs" Select "Mozilla Firefox <version>" in the list of installed programs and click "Remove" to run the uninstaller.
The uninstaller includes the option, "Remove my Firefox personal data and customizations". This will also remove 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.
Even if the uninstall fails, as it may in some cases, continue on with the rest of the uninstall instructions.
Delete the Firefox Installation directory located here, by default: C:\Program files\Mozilla Firefox or, on 64-bit Windows, C:\Program Files (x86)\Mozilla Firefox (Optionally make a backup copy of the "plugins" subfolder first, so that you can restore it later if you plan on reinstalling Firefox.)
(Optional) Enable viewing or searching hidden files and folders (details here) then delete these additional files or folders (recommended if you are preparing for a clean reinstall):
The Firefox folder that contains temporary data ( *.mfl files, Cache and Updates), located here: Important: Make sure you delete the Firefox folder under Local Settings or Local. Deleting the wrong folder will remove your user profile data!
Windows 2000/XP: C:\Documents and Settings\<username>\Local Settings\Application Data\Mozilla\Firefox
Windows 7/Vista: C:\Users\<username>\AppData\Local\Mozilla\Firefox
Windows 7/Vista: The C:\Users\<username>\AppData\Local\VirtualStore\Program Files\Mozilla Firefox folder (if it exists)
Windows XP and above: Delete all C:\WINDOWS\Prefetch\FIREFOX* files
If necessary, remove the "Mozilla Firefox" shortcuts in the Windows Start -> Programs menu by right-clicking and deleting the entries. This step should not be needed unless the Firefox uninstall from the Windows Control Panel failed.
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!
You may also need to manually change some Windows file associations or icons, even after setting a new default browser, as reported here.
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Removing user profile data
Note: 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 XP, click the "Start" button, select "Run", type in %APPDATA% and click "OK" to open the hidden "Application Data" folder, then open the "Mozilla" folder and rename the "Firefox" folder.
Caution: Make sure that you rename 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 in use" or "already running" message when you next attempt to run Firefox (bug 278860).