See this support article:
http://support.mozilla.com/en-US/kb/Changing+the+e-mail+program+used+by+Firefox

This web site will not work for sending a email link. I made firefox 6 my default as shown. I am running windows 7 64bit. I have tried every thing on your web site -its a no-go.

Running FF6 on Vista32bit.

http://support.mozilla.com/en-US/kb/Changing+the+e-mail+program+used+by+Firefox

...is NOT a solution for me. Mailto still broken. Even if I set the mailto option to "Always Ask", it does absolutely nothing. All mailto links simply don't work at all.

No Google Toolbar and now Mailto links not working. What......is going on??? I understood revisions to be improvements but FF5 and FF6 have limited my options and now my productivity is being impeded with these two issues alone.

Very unhappy user.

Fix to enable launching of external mail applications for handling mailto links after uninstalling Google Toolbar:

1. Open the address "about:config" by typing it into the address bar.
2. In the "Filter:" box, type in "network.protocol-handler.external.mailto"
3. If the value of 'network.protocol-handler.external.mailto is "false" (it should be in bold, since this is not the default value), change it to "true" by double-clicking on the name "network.protocol-handler.external.mailto".
4. Go to Options->Applications, enter "mailto" in the search box, and for the Action select the external application you would like to launch upon click of a mailto: link.

Explanation: I was having the same problem as you've described (uninstalling Google Toolbar broke mailto: functionality, and changing the mailto: association under Options->Applications did not fix the problem). After opening about:config, I noticed the network.protocol-handler.external.mailto key was set to false and that it was not being changed despite selecting an external application in Options->Applications. I believe that Firefox should automatically set this value to "true" when an external application is explicitly chosen via Options->Applications, since this is obviously a conscious decision on the part of the user to allow an external app to handle mailto links.

Are y'all trying to drive us to Chrome with FF6? Windows 7 64 bit, Send Link and mailto functions go to default mailer with Chrome and IE8. I followed the instructions in the link given by theedmeister, made sure that Thunderbird was default mailer in both FF6 and computer. Even rebooted.

Nothing. It doesn't work. It's broken. When will it be fixed???

Tried the solution given by ztate (thanks for typing this out), but to no avail. It was set as "true" already, and Thunderbird was, as before, set as the default mailto application.

Mozilla folks - please fix this!

mailto was missing from my applications list, it may have been corrupt, not sure, but this fixed it for me.

Open your profile folder, for details of how to access it see the Profiles article. Now close Firefox, and in the profile folder rename mimeTypes.rdf to mimeTypes.rdf.old

When you re-start Firefox the applications list will be reset to the defaults and should include mailto.

taskmaster76 led me to a fix (thanks!), although not quite as he described.

I started to follow his instructions, and opened the list of applications (Tools --> Options --> Applications). I saw, as before, that mailto was listed, with Thunderbird as the default mail program. But this time instead of clicking on Cancel (because I made no changes), I clicked on OK (just for the heck of it).

Voila! Send link... now brings up the mail program.

This is in Firefox 7.

That worked thanks. When I reset, Outlook was already set as the client (which is what I wanted). I clicked OK and it started to work again. Thank you.

This started when I up dated. to 3.5 I need help I do not know how to do this. This is Fox fire fault.