Firefox sync was working on all my devices as my logins and passwords were perfectly synced. The problem started when I was offered to install the Lockwise app on Android. Where logins were available in the Firefox for Android app they wouldn't sync with the Lockwise app, the syncing went in an infinite loop and did not sync anything. Reinstalling or rebooting did not help. From this moment Firefox for Android offered to fill in the passwords from the Lockwise app, but since there were no passwords synced, there were none to fill. So I decided to uninstall the Lockwise app and go back to Google Autofill service for my global Android password management. But in the Firefox for Android app (which I use the most) I still like to use the build-in Firefox Sync autofill feature like I used to before installing the Lockwise app. Firefox for Android does not offer to fill in any password information anymore. When I look in Tools > Logins all my passwords are there, perfectly synced.
After some research I checked if the following about:config booleans were enabled: and yes they were.
signon.autofillForms signon.autofillForms.autocompleteOff signon.autofillForms.http
Firefox for Android version: 68.2.0