The GPX Viewer script now works in Firefox/Mozilla. With some minor changes to the data format and the script code I have been able to get the equivalent functionality working in Mozilla. Version 3 of the script is now available http://notions.okuda.ca/geotagging/projects-im-working-on/gpx-viewer/.
It also allows for the <html> tag to be with an <extensions> tag which is a more appropriate way to modify the format.
A special thanks goes to Alice Boit for the helpful comments about how I might be able to make it work in Firefox. I didn’t follow your advice exactly, but it got me off my ass to do something about it.
I bashed my head against the wall for a while trying to get the DOM code just right to allow it to work in IE and Firefox until I found this incredibly helpful page… http://www.quirksmode.org/dom/importxml.html.
The next step will be to define a real schema for my extensions.