adb backup -f org.mozilla.fennec.ab -apk org.mozilla.fennec
adb backup restore org.mozilla.fennec.ab
The magic words don't work… Normally that should backup an app, and restore it, however this doesn't work with several major apps, including Firefox.
This comes down to Firefox adding a flag to disable backups
 it's a bit annoying because it's a opt in flag
Firefox, by default you can backup all apps. When you do the equivalent of a
git blame  you see that this was
added to resolve an issue.
It seems to have been a stop-gap fix that got forgotten about, while waiting to
implement a more precise include/exclude list.
Firefox, hurry up implementing the blacklist of potentially dangerous files, or revert the changeset that's doing more harm than good.
Lineage, you should consider ignoring the backup flag, it would be as simple as as dropping line 58-62. This would allow users to backup any app they desire, and help countless users migrate devices.
Google, add a force flag to override the app flag,
adb users are
advanced users, we know we could be copying files that could be device specific.
But we are prepared to deal with that.