Go to about:config in firefox 3
set security.file.strict_origin_policy to false and change the value of security.fileuri.origin_policy to 3 (or 4) - the default is 2.

Details for this option are outlined at http://kb.mozillazine.org/Security.fileuri.origin_policy and http://kb.mozillazine.org/Security.fileuri.strict_origin_policy