I was using Firefox 8.0.1 in Windows 7. (Apparently these instructions would also work for Firefox 3.0.) I was looking at a webpage containing an email address. I clicked on the email link and got a dialog saying this:
This link needs to be opened with an application.
Choose an Application
Hotmail was not on the list. But I wanted to use Hotmail. To find a solution, I did a search and wound up with some advice on the process. I decided to save a writeup of the advice here because it took me a while to find it.
The advice was to type "about:config" in the Firefox address bar, hit Enter, type "gecko" into the Filter box, and double-click on the first item on the list. This was the item called "gecko.handlerService.allowRegisterFromDifferentHost." Double-clicking on it put it into boldface and changed its value from false to true. Then I closed the about:config tab. I went back to the address bar and typed this command, all on a single line:
and then I hit Enter. This gave me a bar at the top with the question, "Add Hotmail (hotmail.msn.com) as an application for mailto links?" I clicked on "Add Application." That gave me a blank screen. I went to Tools > Options > Applications. There were a bunch of items listed there. I went down the Content Type column until I found "mailto." I selected it. This gave me a drop-down menu on the right side. I selected the "Use Hotmail" option and clicked OK.
I went back to the webpage where I saw that email link. I clicked on that link. This opened up the Hotmail sign-in webpage. I signed in. Hotmail immediately started an email addressed to that email address. I composed and sent an email. It seemed to go OK. Problem solved.