Android

I actually forgot that it's more complicated than I made it seem, that's on me. Firefox does make it pretty difficult to add unsupported extensions.

First off, it's not possible on the stable Firefox Android; officially, it's a feature that's currently only available in the Beta or Nightly Firefox branches, which are provided as separate apps in the Google Play Store (or wherever you get your apps). Personally, I'd recommend the Beta build over the Nightly since it's less likely to have severe bugs. Even if you're just using Firefox Android "normally" and don't need it for this purpose, I still think the Beta is an improvement over the stable version because it enables access to less user-friendly parts of the Firefox interface like the about:config page, where you can mess around under the hood. Alternatively, you could use Fennec F-Droid, which is a fork of current Firefox Android which supports the same feature.

If you want to personally pick and choose which extensions you're able to install, there's a solid guide to the whole process you can follow, but you need to register a Mozilla account, create a custom add-on collection and add all of the addons you want to that list, then switch from Firefox's official approved extension list to your personal custom one, at which point you can start installing any of the addons from your list.

If that's more of a hassle than you're prepared to deal with, you could use somebody else's ready-made custom add-on collection to skip out the part about registering an account and building your collection. You don't have to trust me if you don't want, but I just put together a collection of all the officially supported Firefox Android extensions plus Stylus, which you can see here: https://addons.mozilla.org/en-US/firefox/collections/17632282/Plus-Extras/

To add this or your own custom collection to Firefox Beta/Nightly/Fennec, you'll need to

• open to the three dots menu in the browser (next to the address bar)
• open Settings
• scroll all the way down to "About Firefox Beta" or similar and open it
• tap on the "Firefox Browser" logo/text 5 times
• hit back
• scroll back up a little bit to the new (!) Custom Add-on collection button
• enter your custom collection, e.g. 17632282 and Plus-Extras if you want to use my setup from above
• press OK, the browser will now restart

The next time you go to the Add-ons page and forever until you change it (you can make both fields empty to go back to Firefox's official list), it will populate the available plugins list from there instead of Firefox's.