I have tried this on browserinfo, whatsmyua.info and a lot more test sites, not even once have this failed. You set your OS and browser of taste then Apply. You can whitelist pages too, the extensions is on Mozillas Recommended list, cant be better seriously.

Its working flawless! Many thanks to the developer!

Subject.org

Need this extension badly for firefox fenix as on youtube mobile site videos in landscape mode are cut out/cropped/zoomed and works correctly when setting user agent to android 7 and below, means youtube loads differently on android 8 and above causing this issue. Hope you can bring it to fenix.

Does it's job well. Now most sites think this pc is not a Linux machine. However. Permanently changing the user agent to something else. Still could enable sites to identify you. Only thing is, the information you provide is not correct.

Very functional addon. I'm trying this over the other top downloaded user-agent addon by Schlarb due to this one being recommended by Mozilla, but I still think the UI is kinda clunky. Needs faster access to UA switching for specific domains other than black/whitelisting them on the options page.

Work fine!
I use it to regain OLD YouTube layouit and keep my userscripts / Addons working on it.

Wonderful

Youtube 'old layout' army incoming ;-)

Отличная, простая вещь. Главное - работает.

Very nice..

This is a must have since twitter stopped supporting Developer and Nightly Firefox browsers.