I've come across a very odd problem with Firefox, seems like it changes the js functions available when using xhtml+xml doctype. I found that out when trying to use GoogleMaps on a Silverstripe site who served application/xhtml+xml.
An easy fix is to remove the first line on the default template:
<?xml version="1.0" encoding="UTF-8"?>
That will make Silverstripe serve text/html instead.
More Info: http://groups.google.com.gi/group/Google-Maps-API/browse_thread/thread/139dddaa9f726c1b/#