That is not a built-in feature of Firefox, that feature would have been added by an extension. Check to see if there are updates to your extensions, or if any have been disabled due to not being compatible.

Hello, this is a great and sad problem for anybody upgrading to Ff 4.0. A possible solution over the time till the fixing addon is available could be the mailnotifier coming with an thunderbird plugin/extension, popping up even when Ff is opened, or not. The only thing you might do is, to install the "minimize to tray plus" plugin/extension for thunderbird, with the loading with windows option. This will make appearing the small thunderbird quickstart icon in systray, and will be responsible for email notification. By the way the "accountex" plugin prevent you from losing your thunderbird adjustments by exporting them to another partition of your hdd! I hope this might help for a while? PCPero

My second offer to this is the installing of the older version beside the new one: download the older Ff anywhere, and during installation name the localization of the installationfile Ff3, old vers., f. e.. This is necessary for parallel action! Afterwards Ff3 comes up in the well known look, Guxxmail and the others inclusivly! Enjoy it! PCPero PS: All of my clues are depending from my OS configuration, so please do make a backup before using them! I'm not responsible for damages relating to them!