Firefox is not releasing the memory when it is minimized. Opera is very good in this case ;) so here is a tweak for the firefox users to release the memory when minimizing the firefox
Start Firefox
Type about:config in the address bar and hit enter
Type config.trim_on_minimize into the filter field.
If you DO have the preference, make sure it is set to true
If you do NOT have the preference, add it for adding
a. Right click anywhere in the preference list and select New -> Boolean
b. Preference name should be config.trim_on_minimize
c. Select true for the value
d. Close Firefox and then reopen it.