The brand new Firefox for Android rolled out to everyone last month, but it still lacks the broad extension support that made the original browser so popular. To quote Douglas Adams, "this had made many people very angry and has been widely regarded as a bad move." Thankfully, experimental support for sideloading any Firefox extension has now arrived in the Nightly branch.
The current option to install any extensions is specifically for "extension developers and advanced users who are interested in testing for compatibility." Mozilla says the process of installing additional extensions will eventually change, but for now, it's a hidden setting that requires some setup. First, you need the latest build of Firefox Nightly from the Play Store (or APKMirror), and you need to have a Firefox account.
To enable the sideloading option, open Firefox's settings menu, select "About Firefox" at the bottom, and keep tapping the Firefox logo until you get the "Debug menu enabled" message. At that point, a new "Custom Add-on collection" menu will appear in the Settings. This is where you can replace Mozilla's built-in recommendations with any extension collection from addons.mozilla.org.
To create your own collection, log into your Firefox account on addons.mozilla.org, go to the collections page, and click the "Create a collection" button. Here you can search for any extensions listed on the official Firefox add-ons repository. You can also edit the list later, and your changes will appear in the app.
Finally, you have to link Firefox to your custom add-ons collection. Taping the new "Custom Add-on collection" menu in Settings will bring up two text fields: the user ID and the collection name. You can find both by looking at the URL for the collection you made. The first string of numbers is the user ID (12548432 in the below example), and the final text part is the collection name ("corbin-addons" below).
Once everything is entered, press OK, wait for Firefox to restart, and tap the "Add-ons" menu to see everything in your collection. You can install each extension listed, though Mozilla notes that many of them won't work — some APIs are still missing in the new Firefox browser, and many developers haven't even tested their extensions on mobile and/or the updated interface. For example, while Redirect AMP to HTML does seem to work, some of my other favorite add-ons (like Keepa) seem to be totally broken.
- Mozilla Blog