In perspective, it's like a television manufacturer fixing the stream of a particular television channel because it is incorrect. Firefox should not be protecting third party website owners from their mistakes. Second they should not be protecting poorly coded third party plugins. That is why we have the crash protection to begin with! It's the same reason why too many content producers give up with standards because invalid code 'just works'. Where is the incentive to get things right?
Extending this reasoning, if any website takes too long to load, Firefox should simply close the tab, and tell the user that the website has crashed? I guess you're right, that would definitely put pressure on web developers to make sure their sites loaded fast enough to not get rejected by Firefox...but I think this heavy-handed approach is the wrong way to go about it. Pop up a dialog telling the user that XYZ is going too slow, the plugin is hanging, and would you like to kill it? This will let them know why their PC is going slow, but still giving them the choice to continue if they wish. I thought choice was the whole reason people like Firefox, Open Source, etc.
"If I do not want others to quote me, I do not speak." -- Phil Wayne