If you mean to click on a link while in Thunderbird and have it open up Firefox, then this thread put me onto a solution....
here
I modified the procedure abit for my machine and here are my notes...
Code:
1- Create a shell script called "openlink.sh" in /usr/lib/thunderbird-0.6 containing the following lines :
#!/bin/sh
export MOZILLA_FIVE_HOME="/usr/lib/firefox-0.9.3"
url="$1"
if [ "x$url" = "x" ]; then
url="about :blank"
fi
if $MOZILLA_FIVE_HOME/mozilla-xremote-client openURL\("$url, new-tab"\); then
exit 0
fi
exec $MOZILLA_FIVE_HOME/firefox "$url"
2- chmod +x /usr/lib/thunderbird-0.6/openlink.sh
3- Put these lines in the file /usr/lib/thunderbird-0.6/defaults/pref/all.js
// Use firefox for http, https & ftp links
pref("network.protocol-handler.app.http", "/usr/lib/thunderbird-0.6/openlink.sh");
pref("network.protocol-handler.app.https", "/usr/lib/thunderbird-0.6/openlink.sh");
pref("network.protocol-handler.app.ftp", "/usr/lib/thunderbird-0.6/openlink.sh");
4- Copy mozilla-xremote-client from /usr/lib/thunderbird-0.6 to /usr/lib/firefox-0.9.3
Also, to make Thunderbird the default email client, I opened up
Control Center
KDE Components
Component Chooser
Default Component
X Use a different email client >>> Thunderbird