lol, yeh, they did fix a lot of leaks recently, so try updating, but yes, it has always been a common problem with FF, especially crippling with older machines, like PII's etc...
1. Pull up the about:config page in Firefox
2. Create a new setting ([New]->[Boolean]) called: config.trim_on_minimize
3. Select the value True
4. Close and restart Firefox for the change to take effect
by the way you have a lot of things running, 44 is a few too many in my opinion. but i'm used to trying to squeeze the max performance out of lower end computers