Very good extension, but too sad the recent version have stopped working with Firefox. I hope this bug can be fixed if possible. :)

The newest version can be installed on a old version of Firefox (using version 56 myself) by going to this url: https://bugzilla.mozilla.org/show_bug.cgi?id=1549017

Follow the instruction written on the 3rd and 5th comment. 3rd:

"Full Procedure:

Go to about:support in the URL bar.
Open your profile directory.".....

5th comment:

I prefer the temporary fix that I outlined here:

https://www.reddit.com/r/firefox/comments/bkcjoa/all_of_my_addons_got_disabled_and_they_are_all/emggvbx/

A temporary workaround:...

Thanks the ones with the ideas, not me. :)

Missing the "append" mode and a confirmation check when you delete a existing user-agent string.
Apart from that, its fine and simple.

SEHR zufrieden, klappt so, wie ich will.
Manchmal allerdings settings völlig schwarz.

感覺有點耗資源 不過youtube上使用還是可以的

Работает. Но если в настройках видно Linux Firefox десктоп то определяться может как Apple Safari т.е. неправильно. Хотя реально Андроид Firefox. Меняет, но не точно.

good, but lacks list with domains where disabled

Buen

Using Firefox 52.5.0ESR and Java 8 Update 151 ... I can not get this web site to load the crossword puzzle.

https://www.seattletimes.com/games-nytimes-crossword/

It will work in Windows 10 using IE11 and was working just fine on Tuesday Nov 21, 2017
It has been reported to the site manager but they have not responded or corrected the issue.
I tried it using a few choices in User Agent Switcher, but it still did not load. Also, any chance to make
the UAS Button smaller ? ? ? It takes up a lot of room. Thanks

It also allows you to create custom UAs! (Not mentioned in doc but IMO important)