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!