They dropped support for every extension sans 5-6 of them. And some of the extensions were incredibly useful.
https://blog.mozilla.org/addons/2020/09/29/expanded-extensio...
It's just that they're not guaranteed to work yet due to all the changes they made to the browser engine.
https://blog.mozilla.org/addons/2020/09/29/expanded-extensio...
The stable version of Fennec F-Droid (a fork of Firefox for Android) also supports custom extensions through this method if you don't want to use Firefox Nightly: