Tested with FF 18. I have two MySQL servers defined, and the credentials are different from each other. LoginCookieValidity is set to 14400.
Let's say I log in to the first, then choose the second one from the navi panel (for latest git) or from main panel (for 3.5.5) and log in to this one. Then I close all browser windows. Upon opening FF and starting PMA, I have to log in to the first server, but then I choose the second one and bam! I'm logged in.