Pinned tabs are considered as part of the browsing history. A detail like a website remembering you (log you in automatically) is stored in a cookie, so you need to keep these special cookies.

In case you use "Clear history when Firefox closes" or otherwise clear history.

• do not clear the Browsing History
• do not clear the Cookies
• do not clear the Site Preferences

• 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

axelfxz said

Every time I close FF it sign me out from all profiles, delete all pinned "top sites" and closes all the tabs. I just see blank browser like I just installed it, BUT my FF account is still signed in. What to do? Pls help

Thanks for helping! The problem was in "sync settings" I had an options on my phone selected to delete everything, though on pc these options were disabled. Turnin off synch of settings helped here!