Clean install every new version of Firefox, even the most minor of releases. This is very easy to do, and you won’t lose even a tiny bit of your program customization, including bookmarks, extensions, and themes. Here are the steps:
Download the new Firefox update installer from the website and save it in your Downloads folder. (In other words, don’t use the online update method.) Uninstall Firefox from the Add or Remove Programs Control Panel. Restart your computer. Double check that the Firefox program folder in the C:\Program Files folder is deleted. If you installed to a custom folder, make sure that folder is deleted. Do not remove your Firefox Profile folder, located here:
{Drive Letter}:\Documents and Settings\{User Name}\Application Data\Mozilla\Firefox
This folder contains data you need to save to preserve your settings and customizations.
Install the new version of Firefox. When you install, save yourself possible grief and just select the default location for installing the program. And finally, after you launch Firefox and run it successfully, make sure to check for extension updates. Some of your extensions may no longer be compatible (and in most cases, just stop operating) until their program authors offer updates.
That’s all there is to it. It takes less than 15 minutes start to finish, and there are no gotchas or surprises. Each time you perform this step, I recommend that you make a quick review of the extensions you have installed a part of the process, and uninstall any old extensions you don’t use.