Jan Odvarko, the current maintainer of Firebug, is on our team. He's currently the only Mozilla employee dedicated to the Firebug project. He's joined by a healthy community (you can follow the project at https://github.com/firebug/firebug).
Jan and the Firebug team are actively improving Firebug - not just maintaining it so it doesn't break. The bulk of the Mozilla staff (and our volunteer community) is focused on the built-in tools, but a lot of it ultimately benefits the Firebug team.
The standby example is our debugger. A lot of the performance and stability problems Firebug has are a symptom of the old Spidermonkey debugging API (JSD). The developer tools and Spidermonkey team built a new debugging API[1] along with a remote debugging server. The Firebug team is currently working to update Firebug to the new debugging API/remote server.
Most of our work is dedicated to the built-in developer tools. But those also support Firebug development.