I've been looking around for a good image resizing application. This seems to have good reviews. And apparently it works for Canon Raw (.CR2) files as well. I'm giving it a try.
Making Firefox your default browser is not enough. You need to assign the HTTP and HTTPS protocols to Firefox as well. This is how it's done in Windows Vista:
Open Control Panel, Programs, Default Programs.
Click on Associate a file type or protocol with a program.