Does anyone have an idea of how to do it or even if it’s possible? Because as I am paranoid I would like to close my Mozilla account.
BONUS: How to prevent the installation of new extensions from opening welcome pages because it’s very annoying thank you.
Personally, I consider Firefox Sync safe, as your data is end-to-end-encrypted.
If you want an alternative, xBrowserSync exists. Or you can self-host a Firefox Sync server, although this is relatively tricky.
In principle, it is possible to synchronize by just copying files, but with some pretty big caveats:
places.sqlite). So, without writing some custom software, you can’t sync only the bookmarks. And naively copying the history to other devices will cause you to lose the history on those devices.