debugging firefox debug build

I'm interested in learning / watching firefox event handling. I've built 
a debug image and it runs just fine outside the debugger. There was a 
little info on mozilla.org but the links no longer work...

I'm currently adding printf's to a debug build and looking at a log 
file. Is there a better way ? I tried running under the debugger but the 
debugger (ddd) doesn't seem to want to follow new threads. It kind of 
tries to put up the main window and then dies with

[New Thread 113306544 (LWP 14579)]
Couldn't get registers: No such process.
(gdb)

Any info / pointers would be greatly appreciated.