When I close and re-open Firefox, the order of the windows changes randomly. By that I mean the windows that appear when I click on the Firefox icon in the Windows taskbar.
I use different windows for different purposes. E. g. one for the bug tracker, one for the Wiki, one for news, and then some for temporary tasks. Currently I have 11 windows open.
It would be quite helpful if Firefox would preserve the order of those windows. From a user perspective I don't see a good reason why it doesn't do that already.