No, actually it's not. John is part of a team that manages and develops Firebug. The team is still there, including Jan Odvarko who we (Mozilla) pay to work full-time on Firebug.

I don't want to downplay John's part in Firebug, which is huge and it's a real shame that the project is losing him. I just want to point out that you will continue to see Firebug releases going forward.

Kevin (PM for Mozilla's developer tools)