This user agent switcher works well with Firefox Quantum, it modifies the user agent via the HTTP request header and also the JavaScript property navigator.userAgent.
I noticed it does not take effect on https://addons.mozilla.org, I don't know if it is a feature or a bug.

Recommendation: instead of displaying deprecated mobile operating systems like Symbian and Firefox OS, the addon should add Sailfish OS to the list (maybe BlackBerry also).

It would be good to add the possibility to enter multiple custom user agent instead of just one, like this addon (not supported with latest Firefox): https://addons.mozilla.org/en-US/firefox/addon/user-agent-switcher/