Shameless plug: A while ago I wrote a cli for firefox sync [1] (in my case mostly to automate some stuff when I create bookmarks).
It does not really solve your problem for browser-history/credentials etc. But as long as you use a client that you verified you can be kinda sure the data is E2E encrypted and can't be read by Mozilla.
Also on a side note: To this day the weave protocol is one of the more (needlessly) complex protocols I ever implemented and I'm still not sure what problems some of the design decisions solve...
A while ago I did something similar. I'm already downloading various playlists via yt-dlp and wrote a web interface [1] to view/play/search them.
My biggest annoyance at the time was importing my existing videos (and converting them to a streamable format, generating thumbnails and hover-previews etc).
Do you have any plans of allowing to import existing yt-dlp folder (in the standard layout with a bunch of mkv files, the info.json, the subtitles etc). Because my current archive contains a lot of already-deleted videos :(
It does not really solve your problem for browser-history/credentials etc. But as long as you use a client that you verified you can be kinda sure the data is E2E encrypted and can't be read by Mozilla.
Also on a side note: To this day the weave protocol is one of the more (needlessly) complex protocols I ever implemented and I'm still not sure what problems some of the design decisions solve...
[1] github.com/mikescher/firefox-sync-client