When you close down firefox entirely it will forget any temporary cookies. Tabs are the same way, for example you can login to gmail in one tab, and you wont need to login in any of the other tabs.

IE doesn't work that way though, even when both windows are open (you can read 2 gmail accts. at once) IE is a seperate process for each window, Firefox is 1 process for everything. To kill the process, you have to close all the windows.

I wouldn't say this is serious enough for an immediate patch, but maybe a bug fix for any new releases.
http://www.mozilla.org/support/firefox/bugs