Something really weird happens in Vista when I click on mailto links in Firefox (Here's an example of the links that I'm talking about: email someone). When I try to open any mailto links in Firefox, Internet Explorer opens and tries to handle the link. A few seconds later 35 additional IE windows open in rapid succession for a total of 36 IE windows open! While the windows are fairly easy to close since they are grouped together, it's still extremely annoying that 36 windows pop up whenever I click on a mailto link.
How to Fix
I think the problem may have something to do with not setting up Microsoft Outlook even though it's the default email client in Vista. Since I detest Outlook, I'm not going to set it up.
Instead, I installed this Firefox Greasemonkey script to rewrite all mailto links in a webpage to open with Gmail in a new compose window.
In addition, I went into about:config and set the key network.protocol-handler.external.mailto to be false instead of true. This now has Firefox do nothing when you right click on a page and select "Send Link...".