Yeah Leak Monitor works and author is a known figure and contributor to Firefox code/fixes. Think I read some Mozilla Chief Engineer something giving him much credit for 2.0 improvements (bug-hunting I assume). I can tell you many a website/javascript triggered it when I used 1.5.0.x Now on 2.0 it hardly ever happens. Some extensions still make it go crazy. I dont think Mozilla Add-on site should approve extensions which triggers monitor but they obviously do and some extension makers still dont test that much.

I somehow doubt every leak means 500mb used or no one would be able to use Gmail for long but it probably have some negative impact on “memory management”. In some cases may be even stability.