It's a pain debugging Mozilla code, but here's a few quick notes that can help.
Venkman sucks, but it's the best tool you've got. Most of the Nightingale code won't be debuggable unless you have disabled “Hide browser files” in the top-right pane this session. That means if this is the second time you've debugged in Nightingale, you'll have to hide those files, then reveal them again.
Sometimes Venkman will simply refuse to stop in certain code. In that case, putting the line “debugger;” into the code where you want to debug will help.
Venkman can be started from the Tools menu if you have the Developer Extension installed, or by typing “x-jsd:debugger” into the URL bar if you don't. (You might have to be in a debug build for the latter to work.)