1. Backup your bookmarks as a Bookmarks.html file. In Firefox 2 go to the Bookmarks menu, select 'Organize Bookmarks', then select File>Export. In Firefox 3, go to the Bookmarks menu, select 'Organize Bookmarks', then select 'Import and Backup'>Export HTML. Note the difference between .HTML and .JSON Bookmark files is explained in more detail in the Advanced Tweaking section of the guide.
2. Go to Control Panel>Add/Remove Programs in Windows XP; or Control Panel>Programs & Features in Windows Vista, and if a 'Mozilla Firefox' entry (or similar) exists, select it and click Change/Remove and uninstall it. During uninstallation, tick the 'Remove my Firefox personal data and customizations' box as well.
3. Go to your \Program Files\Mozilla Firefox directory and delete it and all of its contents if it still exists.
4. Delete the following directories if they exist:
In Windows XP -
\Documents and Settings\[username]\Application Data\Mozilla
\Documents and Settings\[Username]\Local Settings\Application Data\Mozilla
In Windows Vista -
Note that this will delete all of your bookmarks and saved settings, so make sure you've backed up your bookmarks as covered in step 1. You might also want to note down any passwords which you might otherwise forget.
5. Find and remove all Firefox/Mozilla-related entries in the Windows Registry. Using the Windows Registry Editor (Start>Run>Regedit), delete the following keys - that is, right click on their name in the left pane of Registry Editor and select Delete: