It used to work alright, but sync has been disabled for Firefox from TamperMonkey's end and nobody's been answering the support address in the last month. It just opens a broken OAUTH token window and refuses to sync in either direction. Sync is the whole reason I'm using the extension.
EDIT: Already switched to Violentmonkey because it's open source and has better maintainership that appears to be responsive to users. TamperMonkey should respond on its official contact email and be more transparent about what's happening with the addon.