Sadly on my system X frequently crashes. After a reboot, when trying to
start Firefox again, I get a popup with "Firefox is already running, but is
not responding. To open a new window, you must first close the existing
Firefox process, or restart your system."
Of course, in this case the entire message is not true, as the system is
freshly started, and there are no running firefox processes except those
directly responsible for this message.
The problem is that there's still a .parentlock file in the firefox dir,
from before the crash. Of course, I can simply delete this file and move on,
but I doubt it's that obvious for every Linux end user with no sysadmin
experience.
PS. The same applies to Mozilla Thunderbird and possibly other Mozilla
products. Perhaps this bug should be cloned?