Rated 5 out of 5 stars
I used to use ChromeBug to debug for addons, but it only works with FF5 and lower. Venkman is no longer updated but it still works with FF13.0a2 as I tested. There's some quirks when using it to debug, and its UI is not as well designed as ChromeBug (heck, even the webkit's Developer Tools is inferior to Fire/ChromeBug's UI), so if you only debug JS on webpages, use Firebug instead.
For addon debug though, Venkman still works with the latest FF and helps a lot. Thank you very much! Saves me quite some time.