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.
Does this work with installing xpi files from GitHub? (Say bypass paywalls for Firefox) I remember I used to be able to do this on Nightly, but suddenly I stopped having that ability
Nope. Side loading extensions doesn't work on Stable, Beta or Nightly.
Can confirm, it doesn't work.
You can install ublock origin on android mobile Firefox though, as that extension does work without OP's interesting work around.
Then to bypass paywalls all you have to do is add a custom filter list.