This might be off-topic, but there has been discussion about this issue before on this list.
http://comments.gmane.org/gmane.emacs.orgmode/22550 The problem is that the above discussion doesn't solve my problem. So here it is. On my old system (Ubuntu 10.04, Emacs 23.3, Org-Mode 7.8.02, default browser Firefox, default mail client Thunderbird), I could click on a mailto link in an org file and Firefox would open up Thunderbird. I could then put in the message and send it via Thunderbird. No problems. On my new system (Xubuntu 11.10, Emacs 24.0.92.1, Org-Mode 7.8.02, default browser Firefox, default mail client Thunderbird), an Emacs window in message-mode is opened when I click on a mailto link in an org-mode file. This is not what I want, I want the behavior to be like it was in my old system. My old system just worked without me customizing any variables. I didn't change org-follow-link, org-link-mailto-program, mail-user-agent, etc. I didn't mess around with /etc/mailcap or other stuff. The Emacs variables on my new system are identical to those I used with my old system and the html stuff in /etc/mailcap looks identical. I don't understand why the two systems behave differently. Please don't suggest that I switch to Gnus or something. I don't have time to switch, Thunderbird is easy to set up, it handles HTML well, it reads feeds nicely, and I use the External Editor plug-in (http://globs.org/articles.php?pg=2&lng=en) and emacsclient to write my email using Emacs (as I am doing now). I keep my email addresses in org files and I would like to be able to just click on a mailto link to connect to Thunderbird instead of doing a kill and yank routine.