What do you think this would be worth developed/undeveloped. I plan on a firefox plugin resource site. But obviously there should be a good deal of traffic when firefox 3 comes out in a few years. (unless they call it something else!)
I'd hang on to it and start work on it when Firefox 2 is released. If you have the site design done, you'll be ready to put out news about Firefox 3, screenshots, beta versions, blah blah blah. Of course you're taking a gamble with it being numbered sequentially but browsers typically are so it's highly probable!
If you're going to sell it domain-only wait at least until next year or "Firefox 2" comes out, its value will have at least doubled by then.