Bumped into this on accident this morning but it has a nice web page describing this configuration option. I always set this to 'True' on any machine that is low on memory and poweruser machines.
This works in (I believe) any Mozilla product that uses the underlying core libraries, so for instance, it should/will work in Seamonkey, FireFox and Thunderbird.
I seem to get the best amount of savings with Thunderbird due to my hundreds of RSS feeds.
http://kb.mozillazine.org/Config.trim_on_minimize
