Firefox Accounts lets you add a secondary email address as a backup in case you lose access to your primary email address.

Click the Mozilla account icon on the Firefox toolbar.
In the menu, click on Manage account….
Your Firefox Account page will open.
Next to Secondary email, click the Add… button.
Enter a new email address in the Secondary email text field and click the Add button.
Follow the instructions in your verification email to confirm your second email address.
The Make primary button will now show up under Secondary email. If you want, you can click this button to change it to your primary email address. (You might have to log out and log back in to Firefox Accounts on your mobile devices in order for Firefox to display your new primary email.)

Open Firefox and make sure you are signed in to Sync (click the menu button The image "new fx menu" does not exist. and look for your email address or user name instead of "Sign in to Sync").
In the menu, click your email address or user name to open the Firefox Account panel in the Settings tab.
Click the Manage account link next to your avatar.
Your Firefox Account page will open.
Next to Secondary email, click the Add… button.
Enter a new email address in the Secondary email text field and click the Add button.
Follow the instructions in your verification email to confirm your second email address.
The Make primary button will now show up under Secondary email. If you want, you can click this button to change it to your primary email address. (You might have to log out and log back in to Firefox Accounts on your mobile devices in order for Firefox to display your new primary email.)

Add a secondary email address to Mozilla accounts