Q:

Firefox 4 Downgrades to Firefox 3.6 on Windows 7

On my Windows 7 x64 computer, Firefox 4 has seamlessly downgraded itself to Firefox 3.6 twice.  I haven't determined exactly when this is occurring, but the second time was when restarting after adding GreaseMonkey.  It is possible the first time was also after adding an add-on.  
Re-upgrading to 4.0 seems to work perfectly.
How can I stop this from happening again?

A:

It may be a problem within your profile.
Personally I would backup the contents of C:\Users\Username\AppData\Roaming\Mozilla\Firefox (Just Rename the directory)
Then uninstall Firefox completely, go to C:\Program Files (x86)\Mozilla\ (Mozilla Firefox, whatever it is) and make sure that directory doesn't exist or if it does, check the contents, backup if necessary and remove.
Re-install Firefox, it should be a fresh installation now, give the update a try and see what happens after.

