Yeah it breaks some sites and extensions. There's more info on browser fingerprinting here: https://restoreprivacy.com/browser-fingerprinting/

It doesn't just spoof the UA, it does some other things too. Not sure what they are. It's part of the Tor Uplift[0] initiative.

[0] https://wiki.mozilla.org/Security/Tor_Uplift

Since the generic fingerprint is associated with Tor, you get a lot more captchas. And JS that shows event times based on your clock (say a schedule) will think your time zone is UTC.