It seems that something, possibly antivirus?, is keping thunderbird from updating the prefs.js file at exit. Those are the symptoms. If you view the profile from windows file explorer, you may see multiple copies of that file, representing continued failed attempts. if that's the situation, you might try this: # exit thunderbird # in the profile, delete prefs.js and all other copies except the one with the highest number, and then rename that one to prefs.js # then, invetigate whatever is interrupting thunderbird