This entry will be so brief, it will rival a tweet, but this is pure gold.
If there is any one thing that I found helpful when I needed to debug a Firefox plugin, it was Chromebug. This is the debugger used to debug Firebug, so you know it has what it takes! (yes, I tried Venkman. Fine for some things… just no this) So, get it, follow the instructions and be happy!