Comment 11 for bug 872207

For me using unity launcher panel doesn't avoids the problem. It happens for me if I use multiple desktops. After switching from an desktop on which firefox is not running to firefox by clicking it in unity launcher (also by using keyboard shortcuts to switch desktop or windows) the context menu doesn't work, same for thunderbird. But it's not 100% reproducible it's happen around 50% of the tries to reproduce it.