I have this problem even though I have done absolutely nothing to alter the settings concerned, either through about:config or by editing the prefs.js file. In particular, the network.protocol-handler.app.mailto key was missing from my setup. However I have several other keys that seem to be trying (and failing) to set yahoo.com as the mailto handler (even though I myself do not use yahoo).
I have now created the network.protocol-handler.app.mailto key as suggested below and am about to restart firefox and see what happens. However the fact that I have had to go to an Ubuntu bug report to find out what on earth is going on shows that getting mailto working properly is beyond the capability of the average user. ** Changed in: firefox-3.0 (Ubuntu) Status: Invalid => New -- mailto links in Firefox 3.0b5 do not open up Thunderbird compose windows https://bugs.launchpad.net/bugs/222944 You received this bug notification because you are a member of Ubuntu Bugs, which is subscribed to Ubuntu. -- ubuntu-bugs mailing list ubuntu-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs