Does as advertised, technically. However instead of a menu it's a full page and you have to go in to the extension manager. Maybe they'll update it. If so, don't use the shadow DOM nonsense, so far all UA switchers are terrible.

Super simple add-on to set your own custom user-agent string for all websites. With multiple Firefox user profiles, can set a different string for each profile. It doesn’t work if you have syncing on for add-ons enabled, it will always replace the latest user-agent across profiles.

Improvement ideas for future versions: Ability to (1) have an icon at the top which takes you to preferences, (2) reset the user-agent to browser default (for example with a blank user-agent). Anyway, TY for this!