Works well enough and I haven't noticed performance issues related to it. A few annoyances:
- prevents hitting Enter to submit a login form
- requires being authorized via push notification after every Firefox reinstall (I'm on the beta channel so this happens often)
- no option to just fill in passwords without needing TouchID for every one

Really, this thing shouldn't even be necessary though. Apple provides the APIs for Firefox to directly access iCloud Keychain. Firefox should be using them.