From what I've read Firefox for Android Nightly doesn't restrict which addons are allowed to be installed[1] but does require a really byzantine method to get them installed, by first creating a Mozilla account, creating a collection of addons, then using the collection URL to install them.

It's unfortunate that both being able to install any addon and access about:config on mobile isn't just some checkbox away or at least more accessible without having to install Nightly. A few years ago both were available in the regular version.

One alternative is the F-Droid Fennec fork, which is said to enable both out of the box[2].

[1] https://blog.mozilla.org/addons/2020/09/29/expanded-extensio...

[2] https://reddit.com/r/firefox/comments/wu4g0s/mozilla_firefox...