I've never tried it, but you can probably transfer entire browsing sessions from sessionstore-backups/. I've used those to rescue lost tabs which Firefox failed to recover auto-magically. The compression format is idiosyncratic; Debian has an out-of-the-box tool for working with it — lz4jsoncat from the package lz4json.
user.js is a config file you can put your default preferences into, and clone into new profiles:
https://kb.mozillazine.org/User.js_file
(The stuff you configure in the Firefox UI goes into prefs.js, which has the same format).
Things like bookmarks and browsing histories are sorted in places.sqlite. You can clone/transfer/sync those with whatever sqlite tool.
https://kb.mozillazine.org/Places.sqlite
I've never tried it, but you can probably transfer entire browsing sessions from sessionstore-backups/. I've used those to rescue lost tabs which Firefox failed to recover auto-magically. The compression format is idiosyncratic; Debian has an out-of-the-box tool for working with it — lz4jsoncat from the package lz4json.