To do this, first you need to have Firefox Nightly or Beta installed on your phone, then go to addons.mozilla.org and search in the search bar for User-Agent Switcher and install it by pressing the “Add to Firefox” button and now you should have User-Agent Switcher installed.
Now, you must press the 3 dots on the Firefox toolbar and go to Add-ons and open User-Agent Switcher.
Once there, choose Firefox 117 (or whatever version is available at the time you read this) and it doesn’t matter if it’s Windows or Linux, but to keep things simple choose Windows / Firefox 117.
Now, go to addons.mozilla.org again and again press the 3 dots and activate the option that says “Desktop site” and that’s it, now you should be able to install any addon you want by going to the addon you want and pressing the “Add to Firefox” button, then you will get a pop-up asking if you want to add the addon and simply press yes.
I hope this guide is useful to someone! I recently came across this by chance and really found absolutely no information regarding this, so I posted it here.
It should be noted that not all extensions work as expected, some don’t even open, but most of them work.
Unfortunately the add-on that I want, Yomichan, seems incompatible with Fennec. I’ve done all the collection steps, but I just get a generic “failed to install” message when trying to add it.
Edit: Holy crap. Switching the user agent totally worked. Dead simple. Thank you so much!
Why do they make this so complicated if the add-ons will just install?
Because it is not supported and they won’t offer any help if it borks your device.
The add ons at the moment are developed & tested for desktop not mobile.