(I got this from DP forums, but is is quite usefull)

Hi,
I tested and it worked, that's the steps you should do (Working on version 2 & 3):

Type:
about:config (no spaces)

In the address line and click enter, you'll get a small alert and next step will be to click on the "I'll be careful, I promise!"

Now on the image you'll see above you need to right click with the mouse and then New -> Boolean

Example:


Now you'll have the New Bolean Value, you should type this:
config.trim_on_minimize (again, no spaces - just as it is)

Example:


click OK, and chose TRUE.
Now initialization firefox (close it and re-open)

The result: saving tons of memory and will make the browser faster.


Example:


To this:


It worked from me, I hope it will work for you.

P.S - I didn't knew where to post it since there is no Mozilla forum.

Good luck and let me know if it helped you!