If the Facebook/Twitter integration is only a shortcut for sharing content, wouldn't it be just a slightly different bookmark? I doubt that would bring much more bloat than the bookmark feature? As for the Taskbar web app/Site specific browser thing, that seems to be a HTML-based feature rather than adding website specific code in the browser...
Though I'm using Chrome instead of Firefox because Firefox used to feel pretty slow, so I can't say I completely disagree with the possibility of Firefox being bloated. Does version 4 feel any lighter?