Since a long while now -months or more than a year- I regularly notice that several sites "don't remember me being logged in anymore". This does only happen after a couple of days and not every time I restart firefox. I have no clue what it is, but I think I excluded these options: 1. Firefox visible/user settings. ("Remember history" is my setting, which means remember all cookies until their own expiry date). 2. Those cookies expiry date is set by the sites. Because they expire way too early than before and it cannot be coincidental that suddenly all sites changed their cookie expiration time. (Most sites don't even set it to days to expires, but months or years.) (I'm talking facebook, google, gmail ..) 3. I don't have a user.js in my profile folder. Maybe it's an addon that I installed, but in that case I am not going to disable certain addons for days and then again and again untill I find the responsible, because that would mean weeks of investigation. Isn't there a way to monitor who deletes those cookies after a while? Might it be some Firefox hidden setting ("Here be dragons") ? I díd noticed this: network.cookie.prefsMigrated is set to true, but no relevant settings set. But maybe there are other possibilities than addons? So, to summarize my questions are: 1. Who is regularly deleting cookies? (Generally) 2. If it's an addon, how do I pinpoint to the right addon without trying to find out myself with a huge time investment? 3a. Firefox settings? 3b. Maybe "network.cookie.prefsMigrated"? Is it safe to delete it and try?