[http://wiki.mozilla.org/MozillaQualityAssurance:Home_Page:Firefox_3.0_TestPlan:Leaks:LeakTesting-How-To Leak Testing]
[http://mxr.mozilla.org/mozilla/source/tools/footprint/leak-gauge.html Leak Gauge]
Neat extensions I have found:
[https://addons.mozilla.org/en-US/firefox/addons/display/1320 Gmail Manager]