This addon WORKS VERY WELL with the latest versions of Android Firefox, those based on the new engine codenamed Fenix from 2020 (tested on 121.x to 125.x versions). It works so well that I prefer it over the most popular or usual user agent switchers which can be found in the add-ons website explicitly listed as compatible with Android Firefox.
It's a shame that it's not listed in the menu, though, so it is necessary to find it here and then download and install its xpi file. But this is a different problem, a Mozilla's problem.
Thanks for making this extension, Francesco De Stefano!