Are you possibly using "Clear history when Firefox closes" to clear Cookies and Site Preferences as that removes all cookies including cookies with an allow exception?

• Options/Preferences -> Privacy & Security
  Firefox will: "Use custom settings for history":
  [X] "Clear history when Firefox closes" -> Settings
• https://support.mozilla.org/en-US/kb/delete-browsing-search-download-history-firefox

• clearing "Site Preferences" clears exceptions for cookies, images, pop-up windows, and software installation and exception for password and other website specific data
• clearing "Cookies" will remove all selected cookies including cookies with an allow exception you may want to keep

Clear history when Firefox closes was unmarked (as always) because i prefer clean history manually, never made exceptions, because i never needed, the problem still there (hours ago had it again).

I'm thinking on re-install Firefox and if this don't solve the problem, move to another service since i'm not able to contact with them...

Thanks for try to solve my problem.

If the Firefox settings are OK and you aren't doing this yourself then you may have external cleanup software causing this issue.

If you use external cleanup software (CCleaner) then check the settings for Firefox.

Hi, after some weeks working fine (had to reinstall Firefox again) the problem is back and worse (now close all my accounts while i'm using Firefox)... it's really annoying, my pc have 0 problems, settings are ok too, i only have Adblock+ installed (uninstall is not an option), will reinstall again and if the problem come again sadly i will move to another browser after years using Firefox.