I'd say easily three figures...
Potentially a whole lot more. It appears Microsoft owns the .com and .net even though they are not active. They've owned them since 1996. From that, I'd assume they have some interest in naming a product using them. In addition, I know of at least two major products named firefly. One is a kids phone, and another is an AMR transmitter. I'm sure there are others as well as the obvious insect usage.
Also, the .us version appears to be developed, and the .biz is owned by a major lab company.
I'd definitely hang on to that one. If you find a buyer, I own fireflies.info if they may be interested in a plural version as well.