The old Mozilla Sync for Owncloud is still available, although it hasn’t been updated since Feb 2014. I’m actually running it now under Nextcloud 12.0.3, and it’s still working perfectly.
Unfortunately it won’t work with modern Firefox anymore.
The old style sync (version 1.1) was pretty simple - your encryption key never left your devices and your bookmarks and other data were encrypted before they were sent to Mozilla. Even if Mozilla’s servers were cracked and your data stolen, everything was strongly encrypted.
Unfortunately Mozilla decided to “improve” sync by having it as part of a Firefox Account. This new style sync (version 1.5) requires that Mozilla keep a copy of your encryption key on their servers. They encrypt your key, but the encryption is based on your Firefox Account password, so it’s vulnerable to rainbow tables and other attacks that the key itself is not.
The new style sync (1.5) also requires more moving parts. Whereas the old style (1.1) sync had a sync server and that all, the new style has three interconnected parts - the account server, the auth server, and finally the sync server.
The whole design is more complex (which is why nobody has made a single Owncloud app to replace the old one) and less secure, but it allows Mozilla to tie independent services together into a single login. So far though, there hasn’t been any other services except sync, so the extra complexity and drop in security has been for nothing.
Thankfully, when Pale Moon forked from Firefox years ago, they wisely decided to keep sync 1.1. All my machines run Pale Moon so the old Mozilla Sync Owncloud app is still very handy!
So unfortunately, if you want to sync Firefox you’ll have to build the server yourself outside of Owncloud. It’s messy and fragile, but it can be done. Good luck!