@kaddkaka wrote:
Regarding 3: can I use this is on an air gapped computer or do I need access to some server that is online?
Yes, you can self-host a sync server, although it's fairly technical. The old server (https://github.com/mozilla-services/syncserver) has more self-hosting documentation, but it has been deprecated; I'm not as familiar with the self-hosted setup for the new server (https://github.com/mozilla-services/syncstorage-rs), but there seem to be a few blog posts with recent experience. (Note there are separate syncstorage and tokenserver components you'd need to self-host--I'd probably suggest you read the docs from the old sync server for a starting point.) You'd also need to self-host the mozilla accounts stack (https://github.com/mozilla/fxa/). All this is possible, but fairly complex and not especially well documented; that said, we do have matrix channels where you can ask the developers for help.
If you're not absolutely set on avoiding the public web, note that Firefox Sync has robust privacy properties--your data is encrypted client-side and Mozilla cannot decrypt it; the downside of this strong privacy is that, if you lose your password, your data cannot be recovered. (We derive keying material locally from your username+password and use that to decrypt/encrypt your data locally, and only the encrypted data transits Mozilla's servers.) You can read more about the privacy properties here.
