Firefox normally uses your operating system's default mail program to send an e-mail message. This article explains how you can change the program that is launched when you click on:
By default, Firefox will use your system's default mail client to send e-mail for the actions listed above. To change the default mail client using your operating system, follow the instructions below.
Distributions that use the Gnome Desktop Environment by default include Ubuntu, openSUSE, Fedora, Red Hat Enterprise Linux, and more.
Distributions that use the K Desktop Environment by default include Kubuntu, Mandriva Linux, PCLinuxOS, and more.
Firefox allows webmail services (such as Gmail, Yahoo! Mail, etc.) to enable you to use them for Firefox's mail functions. Use the instructions in the Setting Firefox's mail client section above to use a webmail service with this method.
If the webmail service you want to use is not available in the Applications panel of the Options window, you can use an extension to add support to your webmail services:
Based on information from Default mail client (mozillaZine KB), Change the default e-mail program (Microsoft.com Support), How to change the default web browser or email application (Apple.com Support), The KDE Control Center (KDE.org), and Select Preferred Applications in Ubuntu (DebianAdmin.com).