I already searched a lot in the web but can't find a solution for my problem:

Firefox (since a few versions) does not save any passwords additionally old passwords disappeared. The popup dialogue shows up but when choosing save it does not get saved (also checked the password list which is empty)

To answer the standard questions:
This happens on all sites.
Yes Save Password is checked.
No I'm not using private browsing.
Already reinstalled Firefox

As additional info:
Back when the password function was still working I enabled Sync, on my phone I got all the passwords. (Synced from the PC).
Syncing passwords is enabled but when I manually sync I'm not getting them back.
For troubleshooting I disabled Sync but still passwords are not getting saved.

Maybe you guys here have an idea?