kmozillahelper is only started once per session, therefore multiple windows
share a common pipe to talk to it.

This is not an issue if only one window uses it as a time (only open a single
file dialog, for instance), but there are some other queries that are not
triggered by the user and thus can happen at any time.

Currently neither firefox nor kmozillahelper handle this case correctly, the
protocol needs to be adjusted for this.

kmozillahelper has a workaround to only handle one command at a time, but this
still allows for race conditions on the firefox side. As a result of this a
user needs to close a file dialog before being able to open a downloaded file
or open another file dialog.