Did you ever find a solution?

So far I only got it to work partially. According to this article using a policies.json file would be a somewhat straight forward way to enable the extensions out of the box.

It does indeed work if I install firefox through a linux distributions package manager - it sadly doe not work out of the box when installing firefox as flatpak. So far I tested it for Ubuntu and AlmaLinux. No clue about Windows - haven't tried that so far.

In both cases the installed theme is however sometimes picked up and sometimes not - still trying to figure out why that is happening.