I have recently spent some time hunting down memory leaks in Firebug and the most successful way I have found so far has been analyzing CC (Cycle Collector) object graph - graph of objects that are cycle collected.
The particular memory leak I have been looking for is called a zombie compartment. In my case, there is a document living in the memory even if the parent tab has been closed long time ago. Something is still referencing that document object so, it can't be released. The goal here is usually to find that referent. And this is when the CC heap debugging comes to rescue.