You can only have one default email.

For yahoo it appears to be http://compose.mail.yahoo.com/?To=%s for the link. But the choice come up automatically for me , I'd try sending email to yourself from both Yahoo and from GMail and see if they then show up afterwards as choices in the Tools > Options > Applications > mailto:

For Google it appears to be https://mail.google.com/mail/?extsrc=mailto&url=%s

as seen in details, but I don't see details for Windows Live Mail

When I removed Yahoo! Mail from the list, I thought I could easily replace it (as well as GMail). I was having problems at the time selecting a hypertext email address to respond to someone automatically so I thought I might be able to fix the path for Yahoo! Mail and GMail by removing them and then replacing them.

However, when I removed them, I went back to try to select an application to associate with "mailto" under the Tools-Options-Applications-mailto drop down box by clicking on "Use other". After selecting that, it shows that there are no applications available and allows you to select "Browse" to find one. After selecting "Browse", I do not know what to do to search the web for Yahoo! Mail's correct path. I try to type in the path "http://compose.mail.yahoo.com/?To=%s" in the search box and the file name box and neither work. From there, I do not know what to do.

I tried to upgrade to FireFox 6 from FireFox 5 and this did not replace the web mail applications. Please help!

You should have a read through this. cor-el's notes towards the end are the relevant ones. There must be an easier way but no-one seems to have come up with one .

https://support.mozilla.com/en-US/questions/853283