Firefox dynamically assigns memory cache usage. To reduce memory consumption, you could specify the memory cache capacity yourself, do it by first typing 'about:config' (without quotes) in the Firefox address bar and add a new preference(right click on the about:config page and select new-> integer) and name it as browser.cache.memory.capacity and specifiy the integer value in KB ...however this should depend on the amount of physical RAM you got. For 384 MB RAM this should be between 5500 and 7000. So experiment with the memory cache capacity and see what works best for you. You could also hike it upto 8000 or more and test.
Not to mention, the changes will be reflected and seen when u type 'about:cache?device=memory' (without quotes) in address bar. Here you can see the max storage size, storage in use..etc...
Also, add another preference(if you wish) , right click on about:config page and select new->boolean. type in config.trim_on_minimize as the preference name and set it as 'true' . This tweak is for lowering down firefox memory usage when you minimize it.
Another memory tweak is to reduce memory usage by affecting the back and forward browsing, read about this from here http://kb.mozillazine.org/Browser.sessionhistory.max_total_viewers
Also if you use too many extensions , disable some of them temporarily. That'll reduce some of the memory usage. If you don't know already, disable them by clicking Tools-> Add-ons-> Extensions->Here select the addon which you don't require all the time and hit 'disable'. The addons themselves can pose a memory leak issue. Also upgrade to the latest stable version of firefox.(2.0.0.3). The article about problematic and memory hogging extensions is here: http://kb.mozillazine.org/Problematic_extensions
Finally, restarting firefox after certain periods of time could also help windows recover some of the RAM thereby preventing sluggish performance by your PC and also by firefox.
You can't do anything more than this. Lastly,as an answerer pointed out, the memory leak issue has plagued firefox right from the start.However its getting better in newer builds.
GoodLuck