This howto is about Firefox and Thunderbird and the small changes you need to make to your system in order to get them to work together.

Start problems with firefox

First a hint if you cannot get firefox to start after you have installed it run this comand

xhost +
firefox
xhost -

that should take care of any problems with firefox.

Make thunderbird work with firefox for link handling

Handling of links in e-mails in thunderbird will by default be handled by konqueror (if you have installed kde) to change this you can issue this command and choose firefox in the list

update-alternatives --config x-www-browser

If this does not work then try to add thise lines in your thunderbird prefs.js file:

user_pref("network.protocol-handler.app.ftp", "/usr/bin/firefox");
user_pref("network.protocol-handler.app.http", "/usr/bin/firefox");
user_pref("network.protocol-handler.app.https", "/usr/bin/firefox");

Make firefox understand mailto links and open composer in thunderbird

In order to get firefox to understand mailto links you need to make a file(mailto-handler.sh) contaning the following

#!/bin/sh
url="$1"
mozilla-thunderbird -compose $url

And you need to edit or make user.js in you .mozilla/firefox/xx/ directory contaning the following

user_pref("network.protocol-handler.app.mailto", "/path/to/mailto-handler.sh");

Now you are done when you clik on mailto links in firefox you will open a composer window in thunderbird wether it is running or not.
And if you click on a link in thunderbird you will - in your active window in firefox - load the page or you will start firefox with the link you clicked on.