As of writing this review, the extension pretty much works great.
I'll have to give it 4 stars because the mouse currently doesn't work on the site so only games that don't need a mouse(I tested Brawlhalla and it worked flawlessly) or have controller support are currently playable.

This issue with the mouse is an issue within firefox and not the extension itself but they are working tirelessly on a solution (In that regard, 5 stars would be fair for this review)

Everything but the Mouse works in the Games...
Sadly not playable with every game, since the Mouse doesn't work.