I just tested your extension with this site: https://www.deviceinfo.me/ and surprise it's not that good.
Here is the response from the site "Supports real browser kernel detection even if the browser is spoofed, for: Firefox, Chrome, Edge, Internet Explorer, Safari, Opera, Konqueror."

bruh? this extention has some problems with playing video from Youtube!!!

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.

Maybe I should have watched the YouTube examples at first, but I used to have way simpler, yet in a way better UA-switcher. It supported a short list of Browsers, but the AMO page (and IIRC, the addon itself) had advice to load a large list of Browser UA's categorized. You could pick by selecting type of Browser (Mobile, Desktop), OS (Windows, Linux, MacOS, iOS) and Browser name submenus, so you could easily find an user-agent for example FireFox Fennec (used to be officially named FF Mobile, but since they moved to "FF for Android/iOS" kinda naming, Fennec is what I use to mean FireFox Mobile, which has been from the dawn of smartphones (like Nokia Communicators that 1st supported 3rd party applications - FireFox Mobile and Opera Mobile (not Mini) became available for it. Same for 1-st decade of century Nokia N900 Linux smartphone/tablet OS Maemo (and Meego, which was designed from Maemo. These were full Linux mobile devices, became with built-in capability to download three different lzma compressed tar archives that contained different regular Debian installations that were run under chroot jail (those .tar.lzma archives could be mounted, not just RO, but RW - or it used an overlay filesystem to store additional stuff and edited files (I never was sure if deleting files from original filesystem would be marked somehow on overlay system, so it don't show anymore. But I think tar.lzma can be RW mounted, and grow according to installed and saved work files). But I digress - the point is that the 2 versions that had X-server could run full desktop FireFox (I tried the debian's IceWeasel, which is identical to version of FireFox - they just changed some logo and graphics stuff I understand that weren't compatible with their no-proprietary stuff (nothing incompatible with licences they accept). But it's an ARM device, with like 128, 256 or 512MiB's of RAM - I'm gonna say 256, likely uses zram (which is like ramdisk, but for swap - it uses part of RAM as very fast swap, compressed with zlib, so it kinda can get more out of the RAM. Needless to say that it might even run some post 3.x versions of FireFox, but not even hope to run any of the several dozen latest versions). But I digress...

I didn't find any example of FireFox Mobile for Maemo 5 for example. Same with Opera Mobile. Also, no support of importing standard format list of Browsers in categories like this. - and it was easy to add your own too.

BUT the worst is, I wanted to try out a page with some mobile browesr UA's, but selecting one from list I thought would make it use that. I went to my website, where there is a phpinfo.php file hidden in a subdirectory, which just contains , that command creates a whole webpage, which also lists stuff like what the Browser sent with reguest for the page, including User Agent, and it kept remaining as default FireFox UA - Now I have no idea how to just set it to use a specific UA for a tab, and tabs opened from that tab?

And the supported OS's, Browsers, etc. being limited is a crap Idea - how do I add a browser for Amiga? Or web browser for C-64 Contiki OS User Agent? (yes, it exists, as standalone Contiki application and as part of Contiki OS C-64 version, which implements 8-bit co-operative multitasking and in C-64 (and some other 8-bit system variants) a multitasking windowed desktop that can run web browser, telnet client, screensaver(s), and a bunch of other applications - not simultaneously all of course, but it's incredible how much it can do in 64K's. Why would I want to use that UA? My business, but the old one I used supported adding all kinds of new categories, OS, type of computer (desktop, tablet or phone / mobile, etc.).

It has some more advanced features though, but are they worth what it seems ta lack from the simpler add-on that's support ended with legacy addon support I think.

Excellent switcher with easy selection of browser/os and easy updating of User Agent string. Much easier than accessing about:config.

Unlike other user agent switcher, this one lets you switch on per-site basis!