My hunch is that it happens because dojo.require statements are not handled correctly. It is also possible that this issue is related to another issue reported on the Dojo forum: http://dojotoolkit.org/forum/dojo-foundation/general-discussion/firefox-3-fires-domcontentloaded-event-prematurely.
If you have any insight into this issue, please, let us know. Your help would be very appreciated. Thank you. Happy coding!