Being a Firefox user myself I would also have preferred a browser independent approach, but in the end it had to be IE for the following reasons:

- Active X support is required for smooth real-time interaction between Google Earth and the Globe Glider web view

- Globe Glider should use the web browser window integrated in Google Earth and this window is always handled by Internet Explorer

Anyway, IE is only running inside the Google Earth window and you are free to use Firefox for everything else.