It's not a real problem to do this, but it takes a bit of fiddling. Basically, you go into Firefox's Options, select Download and then edit the action Firefox takes when it encounters a Flash object. On my installation, extensions of "SWF" and "SPL" are listed as being Shockwave Flash Objects, and will open the Shockwave Flash player. All you would have to do is delete these two entries and Flash would no longer play. I haven't found a way to uninstall the Flash player plugin from Firefox, though.