Funny that I used to be that multiple Firebug versions could come up during one version of firefox, now a new Firefox comes out every 6 weeks so you see stuff like this:

Firebug 1.9 alpha for Firefox.next In alpha phase. Compatible with: Firefox 5, 6, 7, 8

Still, the best tool for web developers in Firefox.

Via: WebMonkey

Note: firefox 5 is the current stable, 6 Is the current beta, 7 is also the aurora release and 8 is the nightly.