Firefox optionally syncs a configurable list of things between installations. It uses firefox sync. All of this is opt in and encrypted so that mozilla can't read it.
Cheers, I knew about Firefox Sync (I used Weave with Fennec on the N810 back in the days), I knew it syncs addons, but I did not know it syncs addon settings. I'm still unsure how it determines which settings to use.
https://hacks.mozilla.org/2018/11/firefox-sync-privacy/