This add-on changed my useragent but also broke other add-ons. External video player was one of them..it worked after I disabled UA switcher. Stable Firefox for Android..Note5.
Edit 1. Another add-on that seems to have abandoned stable Firefox on Android..I've noticed the trend. :-\ This works fine, so far, on Firefox beta. Please disregard one half of the 3★s I gave the add-on. It should be 2½★s since it only works on half of the Firefox browsers I use.