In bug 683953 we implemented a global threshold of max. 130 DOMWindows and DocShells kept alive until shutdown. The number of leaks differ quite heavily between OSes so I think we should apply custom thresholds per OS.
These are the current numbers:
Linux (64): 116 (116)
OS X (64): 79 (89)
Windows (XP): 120 (118)
Ftr, what we used to have too for memory leaks was per-application support.
Early numbers for SeaMonkey OSX (64) are: (before bug 730773 detection fix)
"11 DOMWindow(s) and 4 DocShell(s)".
If SeaMonkey numbers are/remain lower to FF ones (which could be likely if there are more FF-specific leaks than SM-specific ones), then I believe we can do without specific support, at least for now.