I dropped the ancient Useragent Switcher for this addon. I like that it's easy for beginners and it keep it's setting after browser restarts.

There's one major annoyance.

There's only 1 version of the OS and browser you can select.

If for example we could pick between Win XP, Win Vista, Win 7, Win 8 and Win 10 and Firefox 2, Firefox 39, Firefox 52 etc.

This addon would be perfect.