To make Firefox use Hotmail for mailto links, do the following.

1. Type about:config into your location bar and hit enter. If you've never edited used about:config before, you'll see a warning.
2. Click "I'll be careful, I promise!" This will bring you to the about:config window.
3. In the filter field type "gecko". Double click the first entry gecko.handlerService.allowRegisterFromDifferentHost to change the value to true.
4. Copy and paste the text between the following text into the location bar and hit enter:

javascript:navigator.registerProtocolHandler('mailto','http://hotmail.msn.com/secure/start?action=compose&to=%s','Hotmail');

5. You will see an information bar drop down at the top of the window. Click "Add Application." You've finished installing the Hotmail protocol now you just need to select it.
6. Go to Firefox>Preferences>Applications, if you are using a Mac, or Tools>Options>Applications, if you are using Windows. Now scroll down the list by content type and find mailto. In the drop-down menu to the right select "Use Hotmail".
7. Next, return to about:config.
8. If the warning comes up again, click "I'll be careful, I promise!"
9. In the filter field type "gecko", and double click the first entry gecko.handlerService.allowRegisterFromDifferentHost to change the value back to false.