browser.xul.error_pages.enabled [Boolean] (True) - If set to true - the default for Firefox 1.5 - then Firefox will displays a full error page when it runs into an error (such as a 'page does not exist' error). If set to false, Firefox will only bring up a small dialog box with the error message, which is the default behavior. I recommend this setting be left at the default of True to assist in troubleshooting web problems.
Looks liek your error, does this help?