They do allow the option to enable flash through the about:config page, they're just disabling it by default. Flash has always been a little shaky on my N810, but it's not really a powerhouse of a device, so I never expected it to handle much more than a youtube video or flash based menus.
Considering FF with Flash on Youtube can bring my well-powered linux desktop to it's knees, I'd be floored if any mobile device could handle it in an even remotely reasonable way.
