I am hard working to fix all Firebug memory leaks (and did fix some of them recently). What I really need is a simple scenario showing that Firebug leaks. That helps me a lot!
Just to clarify, while I am the only one full time developer, there is bunch of other contributors that are doing great work on Firebug!
I am also a member of the Mozilla dev-tools team and one of the Firebug's goals in the future is to reuse Firefox built-in tools and components. This way, we can reduce amount of work on Firebug side and also make sure that APIs of these built-in tools are well designed.
So, Firebug would still be a Firefox extension, but built on top of much better APIs, adding advanced features (e.g. those that are not suitable to be natively in Firefox).
It's also an advantage for other dev-tools that can appear (as Firefox extensions) - especially for those that integrate with Firebug.
Any input is appreciated, please post here: https://groups.google.com/forum/#!forum/firebug
Honza