I had the same headache.
The Error Console is deprecated in Firefox, and is now only made available if you set the devtools.errorconsole.enabled preference to true.
In TB: File->Preferences->Advanced->General->Config editor...
Changes for TB 24)
This is not so clear but i found only this list.
I hope these hits will help you and trust me, I really know how you feel. I also felt this frustration when I had to rewrite parts of my code 1 day before the planned release. :)