On Mac OS X, is there a way to install browser plug-ins (such as flash) for one browser only? That way, I can keep one browser (e.g. Safari) very lean and fast for regular browsing, and another can be fully loaded with all kinds of plug-ins, for sites that require them (e.g. Firefox). Any suggestions or links to related articles are welcome!
Yes, you can.
You could have Safari for speed browsing, and load Firefox with everything you want. They are two completely separate entities that even use a different password remembering system.
Since Firefox is the most likely candidate for loading it with extensions, I'd go with that. Safari is the fastest and most stable browser, so that suits your idea fine too.