Very useful and well working addon, it simplifies debugging of PHP code very much.
I only need two more features in it, I think that with them FirePHP will become THE ultimate debugging addon:
1. The ability to show only FirePHP debug messages in FireBug console. In most cases console is filled with non-critical warnings, but if you disable them to view only useful information, FirePHP output disappears.
2. And the ability to search in Variable Viewer. If variable contains many fields and nested objects, it takes much time to find the needed part, as Firefox search works only in underlying window contents or in Firebug console.
Anyway, great addon and thank you for your work and time!