I've set a custom sync server following this guide -- https://mozilla-services.readthedocs.io/en/latest/howtos/run-sync-1.5.html -- and it works fine for Firefox 70 on the desktop. However, I just downloaded Firefox mobile (latest, v68) and it doesn't work. All options are greyed out, and clicking "sync" changes the text to "syncing", but after a few seconds it returns to the previous state, including "last synced: never". I've set the "identity.sync.tokenserver.uri" value correctly on both desktop instances and on mobile. I have way less than 5000 favorites.