TorBrowser takes too long to shutdown

Originally created by @intrigeri on #6480 (Redmine)

On the tentative 0.22 ISO built on Dec. 9, when trying to close Tor Browser, a Metacity window often pops up pretending the application is not responding and requires to “Force quit”. When asking it to wait a bit more, waiting a few more seconds is enough to see the browser close itself properly.