May be I am stupid but this addon seems like not working.

No clue why Mozilla still recommends this extension. Every time I've used it, it mysteriously disables itself after an unknown period of time. This extension is leaky and it shouldn't be recommended.

Does not work anymore, most sites detect that a user-Agent switcher is being used and also some sites refuse to revert because mobile scaling is not in place. You need to update so that when you select a user agent it also scales the resolution of a mobile phone else the site will not go into mobile mode. Does not work until you get some new user-agents, implement automatic resolution scaling so that the page you visit is in a mobile phone resolution and add anti-detection methods. I think this add-on is dead since the developer is no longer responding which is a shame. Hope someone picks up a project like this and makes a better and updated add-on.

Had the most problems with this extension.
Youtube not loading comments/videos/anything at all , basically a white page.
Another 2 sites giving me a robotic voice when listening to the audio of the videos.
I changed the user agent over 20 times with the hope that I would find one that doesnt cause problems.
And to top it off it was very hard to realise that this extension was causing my problems since 1 out of 10 visits on those websites it would work perfectly.
So i had to clear the cache/data of the web browser , reinstall the webbrowser , reinstall a lot of system components just to find out that this extension was the problem.

The UI is absolutely horrible, shows multiple scrollbars instead of scaling properly to fit the size of the pop-up. The buttons are confusing, there's no clear way of changing the UA of the current tab or resetting it to the browser default (you can only reset it for an entire "container," which is a collection of tabs users are rarely familiar with).

Doesn't work on most sites

Juste parceque j'attends que ce soit dispo sur Android

ERROR[No matching user-agent string for this query]

Первый раз перестало работать.

Useless. Does not work on https://music.apple.com/ running FF75 on windows 10.

This is first time with an Android phone

Not user friendly, Terrible UI. User Can't simply SAVE custom user-agent strings. for standard use, FAQ (its really a "FA-Q") must be read.

Doesn't seem to be designed for easy mobile use

Popup is now completely gone...