For years I had my Firefox sync working fine between Win10 PC, Android and several Win10 laptops. However yesterday I installed Windows 11 on the same PC I have Windows 10 (bought new disk) just to have a fresh install. Started with installing firefox - and here when problems begin. It couldn't let me log in into Sync, I was being immediately logged out ("Account disconnected"). Somehow I managed to log in, but then all my previous data was erased (passwords, bookmarks). I thought ok, maybe later it's gonna sync. Started installing extensions, saved one password. When trying to sync tabs with Android phone it didn't work, it couldn't see any tabs (but it could share tabs between devices!). On Android I still have old passwords and bookmarks saved, same as on old Win10 (on which I got logged out). Alright - it didn't sync after restart so I removed Firefox profile on machine, started again with logging in - and it synced only ONE password that I saved previously on same Win11 machine, with extensions I freshly installed back then. So it saved to sync improper data, forgetting about old? Logged out from all of my machines in Firefox Account. Now I'm back to my original machine on Windows 10. And what? I can't log in! Same error as on Win11 after install - "Account disconnected". Checked about:sync-log and found error logs - pasted into "Troubleshooting information" of this post.

Thanks