Hi Dan

You may be finding that older versions of Firefox for Android are incompatible with more recent updates to the Firefox Sync service.

I would suggest that you use the most recent version of Firefox for Android available from the Google Play store.

Thank you Paul! The last version of Firefox (120) was unfortunately too weighty for Android 5.1.1. I read that there is no support for Android Firefox bellow version 68. I am using currently version 83. With version 105 Sync worked fine. Maybe something in between will work well too.

What do you mean by "too weighty"? The present version of Firefox for Android available from the Google Play store should run fine on an Android 5 device.

That said, Android 5 is about nine years old and outside of Firefox for Android, you may be finding that it is starting to struggle. You may wish to consider getting a newer device, possibly something running version 10 or above.

Yes, the system does struggle with the late FF versions. Exactly what I meaned with "too weighty". FF can not run smoothly any more on Android 5. Anyway, the more recent versions are supposed to Sync without any problems. There is no reason, why the shouldn't. Except the connections to the servers change with each update!

The connection process is subject to updates to make sure that it is secure and performance. I recommend using the latest version of Firefox for Android available from the Google Play store.