Firefox 4.0 reopens the last webpage visited if the cookies window is open.
See also related problem: Firefox 4 doesn't open in a homepage, but rather opens the last page I visited, which I HATE!!!!!!!!!!!!!!!!!! http://support.mozilla.com/en-US/questions/789325
If the Cookies window is open and no browser or other Firefox windows are open, opening a new browser window will now still open the last page visited instead of a blank page, even if "Show a blank page" is selected for "When Firefox starts:", and even if the last browser window opened was blank; I tested this by opening a webpage and then opening a blank browser window, closing the browser window with the webpage, then closing the blank browser window, then opening a new browser window while the Cookies window remained open. It is a time waster to have to continually close and reopen the Cookies window and a potential privacy disaster to have your website reopened after you closed your browser. Please, please, fix this.
Note: the Options dialog has to be closed while doing this.
It is also a waste of time to have to re-clear cookies, and flash cookies as well, when the webpage reopened when you weren't expecting it.
Work-around: keep a blank browser window open while keeping the cookies window open.
See also related problem: The Firefox 4.0 cookies window does not update while open any more: https://support.mozilla.com/en-US/questions/801527