Aaron suggested having an "about:memory" page showing a breakdown of Firefox's memory use (bug 392351). When I pointed out the bug to Brendan Eich, he excitedly assigned the bug to himself.
Robert Sayre created a script to load random pages and see whether they cause leaks. The random URLs come from the Yahoo directory (biased toward older, top-level pages), del.icio.us (biased toward newer, geeky pages), and AltaVista (biased toward pornography).I see they have their use cases covered.
Steve England tested the top 500 web sites, finding two leaks. Later, he tested the top 20 Firefox extensions and found leaks in several of them.And there are some interesting user comments on the page.
Could I suggest a test of a 10 minute session of scrolling and zooming around in google maps hybrid mode as something guaranteed to to eat over 1GB of memory?I'd venture to say, from personal experience, that Yahoo! Maps (which appears to be a clone of Google Maps, and and is, to the best of my knowledge, only used for geotagging photos in Flickr) appears to chew up more memory than Google Maps. Which is rather funny, what with Yahoo! employing some of the brightest minds in AJAX development today (Douglas Crockford, for one).
Anyway, good luck to the Mozilla developers. Speaking as one in the habit of leaving lots of windows open in a session, I hope that this will lead to a browser that doesn't chew up all of the computer's resources if used for more than a few hours.