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TweakGuides.com: "Uninstalling Firefox

To ensure an absolutely trouble-free experience with the latest version of Firefox it is always recommended that you uninstall any previous version of Firefox completely before installing a new one. To completely remove all traces of Firefox from your system follow these steps:

1. Go to Windows Control Panel>Add/Remove Programs, and if a 'Mozilla Firefox' entry (or similar) exists, select it and click Change/Remove and uninstall it. Reboot if required.

2. Go to your Documents and Settings\[username]\Application Data\Mozilla directory and delete the entire \Firefox subdirectory and all of its contents. Note that this will delete all of your bookmarks and saved settings. If you want to backup any or all of your current Profile, see the Customizing Firefox section first.

3. Go to your \Program Files\Mozilla Firefox\ directory and delete it and all of its contents.

4. Using a registry cleaner like RegCleaner 4.3, find and remove all Firefox/Mozilla-related entries (usually there are several Mozilla entries to be found) and remove them. Alternatively you can use the Registry Editor (Start>Run>Regedit) and delete the following keys - that is, right click on their name in the left pane of Registry Editor and select Delete:

[HKEY_CLASSES_ROOT\FirefoxHTML]

[HKEY_CURRENT_USER\Software\Classes\Applications\firefox.exe]

[HKEY_CURRENT_USER\Software\Mozilla]

[HKEY_LOCAL_MACHINE\SOFTWARE\mozilla.org]

That will remove the bulk of the Firefox-related settings which can cause problems with newer installations if they are corrupted for example.

In most cases you should only have to undertake Step 1 of the above list. However if you are having any problems with Firefox, or if you just want to be cautious I suggest you follow all the steps above. Once again if you need to backup any of your bookmarks or settings first, read the Customizing Firefox section below before following the steps above."