/usr busy after aptitude operations?

["Todd A. Jacobs" <nospam@codegnome.org>]

I have the following in /etc/apt/apt.conf:

    Dpkg::Pre-Invoke {"/bin/mount -o remount,rw /usr";};
    Dpkg::Post-Invoke {"/bin/mount -o remount,ro /usr";};

However, in many cases, running aptitude results in the following error:

    mount: /usr is busy
    E: Problem executing scripts DPkg::Post-Invoke '/bin/mount -o remount,ro /usr'
    E: Sub-process returned an error code

I can't figure out why aptitude is leaving the partition in a state
where it can't be remounted ro, even after a significant amount of time
has passed or /bin/sync has been called.

Running "fuser -vm /usr" reveals that /usr/lib/iceweasel/firefox-bin is
being left open for writing (why???), but even after closing firefox and
seeing /usr/lib/iceweasel/firefox-bin disappear from the fuser output,
the problem remains the same.

How do I work around this issue, or at the very least find out why the
filesystem is being held open?

-- 
"Oh, look: rocks!"
	-- Doctor Who, "Destiny of the Daleks"