How can I configure my browser to allow JavaScript-invoked printing of an iframe from a different domain?

I have a particular application that needs to print arbitrary webpages sent from a server. I have a setup with Firefox 3.6 that allows me to do this by loading the page into an iframe and then calling printFrame.contentWindow.print() as an onload() listener. I work around the cross-domain restriction by setting the "capability.policy.default.Window.print" userPref to "allAccess" (and also work around the print dialog problem by setting "print.always_print_silent" to true).

However, when I try to upgrade to the current version of Firefox, this system no longer works. The "print.always_print_silent" setting is honored (I don't get print dialogs), but "capability.policy.default.Window.print" seems to be ignored; I can print URLs from the original domain, but URLs from other domains fail.

Is there a new setting that will enable the same behavior as "capability.policy.default.Window.print" used to? Or some other way to tell my Firefox installation to print an aribtrary URL without user intervention?