I use the official binaries of Firefox and Thunderbird from Mozilla. With this arrangement, hyperlinks in Thunderbird do not open into Firefox (the only browser on my system). It is inconvenience to copy each URL I receive in an email and paste it into Firefox. To solve this problem I exited out of Thunderbird and created the user.js file in my ~/.thunderbird directory (this file is found in a directory with a random string of numbers followed by .default). I edited user.js to look like this:

user_pref(“network.protocol-handler.app.http”, “/opt/firefox/firefox”);
user_pref(“network.protocol-handler.app.https”, “/opt/firefox/firefox”);
user_pref(“network.protocol-handler.app.ftp”, “/opt/firefox/firefox”);

Now I opened up Thunderbird and clicked on a Hyperlink in one of my eMails and Firefox promptly opened up and sent me to the URL. It seems that this behavior is default in Windows, but us Linux Users, who may not use the official packages provided by our distribution might have to make this adjustment. Alternatively you can use the ThunderBrowse plugin which will embed a simple browser within Thunderbird, or can be configured to specify an external browser.