Please let me know if any of these combinations do not work or if the 
extensions auto-update to the wrong versions.
As for issue: http://code.google.com/p/firephp/issues/detail?id=174
The incorrect Firebug compatibility check when running 0.6.1 with 
Firebug <1.8 has been fixed with the 0.6.2 release which can be 
downloaded from the front page.
If 0.6.2 works out ok on the RC channel I'll push it to AMO ASAP. Please 
confirm that the 0.6.2 release fixes what is broken in 0.6.1 and 0.6.0.
Sorry about these compatibility issues. To make things better for the 
future I have moved the FirePHP extension project to github [1], 
refactored it to run in-place from source for easy hacking and tied it 
into a new build and publishing system so I can push releases much easier.
Christoph