Your ‘logged-in’ status is stored in special cookies. If the cookie is removed, you are logged out.

Type about:preferences#privacy<enter> in the address bar. The button next to History, select Use Custom Settings.

Turn off Always Use Private Browsing Mode Turn on Remember My Browsing And Download History At the bottom of the page, turn on Clear History When Firefox Closes. At the far right, press the Settings button. Turn on ONLY Cache and Form And Search History leaving the others off.

FredMcD said

Your ‘logged-in’ status is stored in special cookies. If the cookie is removed, you are logged out. Type about:preferences#privacy<enter> in the address bar. The button next to History, select Use Custom Settings. Turn off Always Use Private Browsing Mode Turn on Remember My Browsing And Download History At the bottom of the page, turn on Clear History When Firefox Closes. At the far right, press the Settings button. Turn on ONLY Cache and Form And Search History leaving the others off.

Thank you so much.

Hello,

I am glad to hear that your problem has been resolved. If you haven't already, please select the answer that solves the problem. This will help other users with similar problems find the solution.

Thank you for contacting Mozilla Support.

Hi Jettugi, did you install the Facebook Container add-on around the time this happened? A couple of users have reported that their cookies were removed when they did that, but it might be a coincidence.