I had an addong called useragent switcher before and this aint it! Wish I could find it. I don't know what to do in this current version. I used the button on the tool menu and selected a newer version of Firefox but when I click test it still shows my actual version--52.5 ESR and it doesn't fool the addons site when I try to install a new addon. So far none of the user agent addons are working, so adding to the list of fails.

Can you spoof UserAgent from header ?
Check on https://browserspy.dk/useragent.php
Not working on first visit.

So far it is working well. Thanks a lot! Finally i am out of request desktop site hell.