I've been working on this for a while.
Sadly, newer versions of firebug are not compatible with the gecko
version used on SeaMonkey 1.1, as I don't use it anymore, and it's near
its end of life, I left 1.1 alone.
There are some other required changes, but most of it was already done.
I'm still waiting for a version of Firebug fully compatible with Firefox
3.1 branch (and so, SeaMonkey 2.0 branch).
I expect when 1.4 is released I'll publish it.
I made a version for 1.3a2, and since then I had no time to update it.
It works, but it is not stable.
For SeaMonkey users, the best is to wait until SeaMonkey 2.0 is released.