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."

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.

with this extension the open container tab context menu appears


why? this is so annoying, theres no need for it, other than that it spoils a perfectly good add-on

Confusing UI/UX results in mistakes. Why not just make the button image correspond to your useragent instead of the useless logo. I am glad they finally added support for container but now i can no longer set default user agent for all tabs or on a per-window basis. Coupled with Temporary containers addon this makes the addon useless since i have to manually set the user agent for every new tab. Also Shows ads for other addons in the dialogue box.

The only reason i use it is because it is a Firefox "recommended" addon.

Doesn't seem to be able to spoof the Operating System...

see https://www.whatsmyua.info/

Appreciate it, but testing websites are still able to detect my real browser in what they call the "core" browser being used. Hopefully something can be done.

No graphic way for adding domains

Incómodo de usar, dependiente de Internet para buscar en una base de datos que, encima, está desactualizada.