Yea, it's pretty easy if you already have a server. All you need to do is run a docker container, and change the identity.sync.tokenserver.uri setting in about:config. On mobile you have to enable the debug mode by going to "Settings > About Firefox" and tapping the Firefox logo a few times, then go to the new "Sync Debug" settings entry.

The container above only runs the sync-server though, you still have to log into a Mozilla account to use it. There is a replacement that includes the whole stack, but I haven't tried that one yet.