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You would probably start with one basic stylesheet and then introduce the ones with specific tweaks further along the cascade.
The conditional comment aprpoach was invented by IE because their crippled browser required it so often. firefox is most often quite well behaved so there is no proprietary approach that I know of. There's a very thorough Browser Sniffer post in this thread: [webmasterworld.com...] - that's from 2001, back when we really needed it.