Tbf, Mozilla isn't about Firefox anymore. They are focused on privacy everything, and are apparently going to do lots of important and foundational work in AI because it's the next new frontier (and they're right).
They aren't going to abandon Firefox, far from it. As the only other competitor to Chromium, they're going to continue to promote privacy online, but they're not going to make Firefox their core development either.
Just install decky loader, install MangoPeel, and you can configure the overlay in the plugin settings right in your steam deck.