changing default application associated with each file extension

[Jeff Co]

Hello, 
I use Gnome and Nautilus. How can I change the default application
that opens up when I double click a file in Nautilus? For example,
html files open up in Firefox, but I would like that to be changed to
Galeon. I don't want to bother with having to right click the file,
then selecting "Open With..."

Thanks.
jc