Good news: I've got to the bottom of the problem.
Sending referers had been disabled in Firefox. As I cannot recall disabling the function, it may perhaps be the new default introduced by version 4 of the browser--I will check.
In case anyone else is experiencing the same problem, the method described below is how I enabled/re-enabled sending referers in Firefox. Another method is to use third-party software to manage and control the sending of referers.
in the URL address bar.
Look for network.http.sendRefererHeader
in the list of preferences
Right click on network.http.sendRefererHeader
and click Modify
Set the value to 2
. (0 disables sending referrers)
More info from Mozillazine: http://kb.mozillazine.org/Network.http.sendRefererHeader