Ah I see, on Firefox mobile, one cannot load extensions from their website, only from some extensions manager inside the app.
https://blog.mozilla.org/addons/2020/09/29/expanded-extensio...
Alternatively, you can use Fennec F-Droid, a fork of Firefox that supports a "Custom Add-on collection" by default: https://f-droid.org/en/packages/org.mozilla.fennec_fdroid/
I'm not sure why Mozilla is refusing to allow these extensions on the Beta channel, since the WebExtensions support is definitely stable enough for Firefox Beta.
Heck, I can only partially understand the "click many times over crap" to enable experimental features, which should have been _enough_ to begin with (and certainly not a requirement in a beta build). How many hoops do I need?
After reading this, I was really /hoping/ Fennec would remove the requirement of having a Mozilla account, but no, you still need to create an account and an useless "collection".
TBH I use (and ever used) Firefox just due to the available extensions. The value proposition has decreased quite a bit with recent FF versions, and on Android uBlock is pretty much the only reason I stick to it.
I wish I had more alternatives, because clearly the browser duopoly isn't working.