Sometimes an application gets in trouble with unity and then it won't be shown in the foreground anymore. To get to the application I have to minimize all other windows. Alt-tab doesn't bring the app to the foreground. This has happened with Firefox and Thunderbird.
Killing the app and restarting it doestn't help. It stays in the background after restarting it. Other symptoms:
The app doesn't have a menu bar anymore.
The white filled triangle transforms into a white arrow as shown below.
Only restarting the computer fixes the problem.
Because of the white arrow I think this might be a feature instead of a bug but I don't understand it. How can I undo this behaviour? I have spent about 4 hours googling it but there is not much to be found on unities triangles or applications that stay in the background.