Perhaps there are still some minor bugs and they don't enable it by default?
And if you zoom in somehow, you cannot pan over the page because Firefox act like you are seeing full screen and trying to go back or forward in browser history.
Need to first pan at least a bit vertically, then it allows horizontal panning. But yeah, clearly not a finished feature yet. Still nice to see there's progress, and it's already mostly usable with the mentioned caveats.
Also read the description: there's an option to enable it, but for me at least, it was too buggy to use.