Firefox Accounts now lets you change your primary email address if you have two emails registered.

1. Open Firefox and make sure you are signed in to Sync (click the menu button and look for your email address or user name instead of "Sign in to Sync").
2. In the menu, click your email address or user name to open the Firefox Account panel in the OptionsPreferences tab.
3. Click the Manage account link next to your avatar.
4. Click the Change… button next to Secondary email.
5. Click the Make primary button.