This is the instructions I was looking for, but thanks for your help.
First, close Firefox. It shouldn’t be running when we’re editing these files.
Look for your Firefox profile folder. It’s probably at /home/[user]/.mozilla/firefox/default, but it may be elsewhere. If you’re not sure where it is, then follow this link, which leads to the excellent MozillaZine Knowledge Base and should tell you what you need to know.
In your profile folder, you may see a file named user.js. If you do, great - we're going to be editing that file. If you don't, then create a file named user.js using your favorite text editor.
Open user.js and enter the following line: or copy and paste is the best method.
This is key: if you don't want to use Thunderbird as the mail program that opens when you click on a mailto link, or if you've installed Tbird in a non-standard location, you need to change /usr/bin/thunderbird to a different path. In my case, I used /usr/bin/kmail. (If you use pine and want a brilliant solution to this problem, read this message at NewsForge from someone who got pine to work - use user.js instead of prefs.js, however).
Save user.js, restart Firefox, and find a web page with an email link on it. Click on the link, and an email ready to compose, with the email address already filled in, should open using your preferred email program. Cool.