It's not in the official Firefox Add-ons for Android. I went to the Nos2x Firefox fork repo but Firefox wouldn't let me install it there either. Tried downloading the signed xpi and no go.
Did you get this working?
Discussion
I'm pretty sure you can sideload any extension, you might need to change something in the "hidden" advanced settings first though.
I just use Onyx so I've not done it myself yet but I will do when I can get onto Mutiny.
If you look up sideloading Firefox extensions on Android I'm sure there's info on how to do it. Firefox for Android is pretty much a full port of the desktop version so it can do it, just a matter of letting you install extensions from outside the official site.
Use Onyx or mutiny? Are we talking about the same thing here? I was wondering how to sign Nostr messages securely in a mobile web browser.
Did you try the non forked version?
That is the fork of nos2x and it doesn't work in mobile Firefox unless I'm missing something.