There was an update in the firebug version which fixes a couple of issues and covers a 0-day security hole.
The update has been published to addons.mozilla.org, so you can get it by updating Firebug from the Firefox Add-ons window. Alternatively, you can install the update using the big orange button on the firebughome page.
Update -- go get 1.04 -- its a more robust fix for the security issue