Clicking on the Applications menu, selecting Desktop Preferences, then selecting Advanced, and then selecting Default Applications.

As of 2018, in openSUSE ( GNOME 3.28) it's under System menu > Settings > Details > Default Applications.

(Ubuntu) Clicking on the System menu, selecting Preferences, and then selecting Default Applications.

The Default (Preferred) Applications window can be opened by:

Open the Default Applications (formerly: Preferred Applications) window.

Gnomeĭistributions that use the Gnome Desktop Environment by default include Ubuntu, openSUSE, Fedora, Red Hat Enterprise Linux, and more. Your distribution may use a different interface. Note: These instructions are for some common Linux distributions. Any changes you've made will automatically be saved.

Application Details… will let you view information about or remove the webmail services that Firefox can use.

Use other… will let you specify an external program for Firefox to launch.

For more information about using webmail services, see the Using webmail services section of this article.

Use will open the message composition page for your webmail service (such as Yahoo! Mail or Gmail) inside Firefox.

For instructions on how to change your operating system's default e-mail program, see the next section of this article.

Use (default) will automatically launch your operating system's email program (for example, Thunderbird, Outlook, Windows Live Mail Windows Mail Mail Evolution, KMail) to its message composition window.

Always ask will prompt you each time you use a mail function for which program or service to use.

Click on the Action column in the mailto row to change the action.

Search for the Content Type mailto and select it.

In the General panel, go to the Applications section.

Click the menu button and select Settings.

In the Menu bar at the top of the screen, click Firefox and select Preferences.

2 Setting your operating system's default mail program.

1 Setting the mail client used by Firefox.

Firefox email app