I remember using extensions on Firefox android years ago. Is my memory failing in this or did they remove them before putting them back?

Your memory is failing you. Been using firefox android ( stable and beta) with the addons (ublock etc) since a few years now.

You could install custom extensions on Firefox Android. Now you have to jump through hoops to install anything that isn't blessed by Mozilla.

Indeed. They have too much authority with the marketplace. Does someone know if it is feasible to build Fenix from source, fork it and then add a startup function that sideloads any extension the user chooses, not just the blessed ones? I'm considerung doing this, but don't want to spend a lot of time doing something impossible.