Make sure that you use a Firefox Profile Folder with only one Firefox version. If you switch versions then Firefox will perform a compatibility check on the next start.

This can also be caused by a problem with the file prefs.js that stores the preferences.

• http://kb.mozillazine.org/Preferences_not_saved

I renamed the profile and Firefox started another profile but it did not work - the "Checking compatibility of add-ons" comes up every time I open my Firefox browser. I have even unchecked "Check for Firefox Updates" but that did not stop the pop-up.