I use ff 42 stable and I don't know where this problem came from. Maybe I had installed harmful extension or something. Fullscreen mode error (occur with my signed-firefox account profile) Every times I leave firefox in fullscreen when exit. Few restart later, Firefox didn't hide its toolbar as usual but this time the close, restore button on the right concert, bookmark items disappeared and I got a blank space above Tab bar. But Mozilla.org can be loaded normally, accounts.firefox.com is accessible (in my old and signed profile). Mozilla.org cannot be loaded normally, accounts.firefox.com is inaccessible (cannot sign in) (occur with new profile) I thought the old profile was broken so I tried to sign in my firefox account with the new profile but the sec_error_unknown_issuer error suddenly appeared.The worse came to the worst, when I tried to access Mozilla.org (support page, addon page..), the web was just a white paper with simple text. Oppositely, with this new profile fullsreen mode work perfectly. I have tried many times, disabling all extensions, going back with default theme, uninstalled ff by Revo Uninstaller and reinstall, restart ff mutiple times, refresh the old profile... but unfortunately, the problem hasn't been solved. And I was taken aback to know that Mozilla.org cannot be loaded normally, accounts.firefox.com is inaccessible also happened with firefox portable (portableapps.com) and firefox 43 beta. Could you help me with this? Regards.