I know its an older version of TB, but new versions did not help me and I prefer some add-ons (expression search) that don't work in newer versions.
I faced similar issues and found a few nuggets in the various replies that had me looking at potential causes. Its not really a leak as it was (for me) quickly up to 750MB of memory usage and then somewhat sable (but would jump beyond a GB on a regular basis). I found the steps at to
https://wiki.mozilla.org/Thunderbird:Te ... e_Problems helpful and worked through them one at a time and measure the impacts. (I had disabled the global searching a long time ago, and don't use newgroups in thuderbird, many of the ideas here do not apply).
For me the killer was big folders. After reading about it and installing the foldersize add-on (not in the searchable list of add-ons, but on the memory_usage_problems page), and using it to identify (and the using move reduce folders that were above 2GB). In my case there were about 20 of them and at least 6 were very close to 4GB. After that cleaning my tbird as back to a far more reasonable 130MB of private storage.
YMMV, but of all the things I tried, this was both easy to do and far and away the most impact. Too bad it was #14 on their list of thing to check, as I examined many other things that did not help my machine performance.
MAC Book Pro, OS 10.6.4