As much as I like Firefox..this decision was brain dead. Firefox caches your tabs data in memory. It then tries to cache 4 -8 levels deep PER TAB in ram. I have watched Firefox use 250 megs or more. I found two tweaks. Both of them chop the amount of memory Firefox uses. Unless you are on dialup you do not need memory caching of webpages and linked pages. Here’s how you kill it:
in your address bar type about:config
double-click it and change the number to zero.
The next one further reduces Firefox’s piggishness:
find the setting browser.cache.memory.enable
double-click it to switch it to false if it says true.
next mission..pare down thunderbird’s ram usage.
*UPDATE* the browser.cache.memory.enable setting turned off it turns out is breaking scripts that apparently MUST cache results into ram. Use this setting with caution.