Firefox Accounts now lets you change your primary email address if you opt in. Follow these steps to opt in and enable this feature.

1. Open Firefox and make sure you are signed into Sync (click the menu button and look for your email address instead of "Sign in to Sync").
2. Go to https://accounts.firefox.com/settings.
3. Log in to Firefox Accounts.
4. Under Secondary email, add another email address and click the Add button.
5. Follow the instructions in your verification email to confirm your second email address.
6. The Make primary button will now show up under Secondary email. Click the button to change this to your primary email address.