Warhammer 40K would make a good MMOFPS, especially a point-control one like Battlefield. Make it a persistent campaign world or worlds where the individual battles have an overall effect on the game story (I think it was Planetside that did this, but I have no idea how good it was). But it would make a lousy MMORPG IMHO because you lose a major feature of the universe: that individuals are faceless cogs in the larger machine. Any RPG, MMO or single player, sets up the character as a person of note and importance. In 40K 99.999999999% of humans are completely unimportant.
I'm not sure how well Firefly would do as an MMO. The universe is certainly varied and interesting enough, but they would have to significantly change the feel of the game from that of the show. In the show, everything revolves around this small group of people living and working in a cramped space. That would pretty much have to go out the window if they wanted to make a traditional MMO. The only way you could keep the flavour of the show would be to make it a highly segmented online game. You could divide the population into two major groups: spacers and planetsiders. The planetsiders are divided into factions that are struggling against each other, and they recruit/hire/manipulate the spacers into doing jobs. The spacers can take jobs for money or other rewards, or they can raid/steal to get what they need. Make it so that the spacers would have little interaction with people outside of their own crew. Essentially it would be two games melded together. Actually, that sounds kind of sweet. I'd play that game.
As far as other universes go, I second the suggestion of RIFTS. The RIFTS universe has a ton of varied areas that just scream for loads of expansions. Start with a general Coalition States/Tolkeen/Free Quebec and points between, and you can expand to the New West, Xiticix Hivelands, Federation of Magic, Vampire Kingdoms, and the East Coast. And THEN you have the entire rest of the world from there. There's tons of room to join the Coalition (technology), the Tolkeenites (fantasy) or go independent. Boatloads of races and classes to choose from as well.
Another that might work is the Deadlands universe. An Old West setting, but with the addition of many fantasy elements, and a healthy dose of steampunk to go with it.