Setting the browser that opens Link in Thunderbird - in Linux

To open the links in browser from thunderbird follow the steps below :
Step 1: Open "Edit -> Preferences", Click "Advanced" icon.

Step 2: Click "General" tab, then click on "Config Editor". Proceed through the warning by clicking on "I'll be careful, I promise!" button in the "about:config" dialog.

Step 3: In the Filter search for the following and update as given below,
to update right click on the entry and select modify for the following entries,
i. Preference Name: network.protocol-handler.app.http Value: /usr/bin/firefox
ii. Preference Name: network.protocol-handler.app.https Value: /usr/bin/firefox
iii. Preference Name: network.protocol-handler.app.ftp Value: /usr/bin/firefox

to update right click on the entry and select toggle for the following entries,
i. Preference Name: network.protocol-handler.warn-external.http Value: true
ii. Preference Name: network.protocol-handler.warn-external.https Value: true
iii. Preference Name: network.protocol-handler.warn-external.ftp Value: true

Step 4: Click Ok and close the about:config and then the Preferences window.

Note: Now when you click on a link in ThunderBird, it will prompt you to choose the application with which the link has to be opened. Choose you application and set as default by checking the checkbox.