If you – for some reason – don’t like the yellow (for contacts in your address book) or gray (for unknown contacts) star next to email addresses in the message header there’s a simple way on how to hide it.

If you don’t have one yet, all it needs is a file called userChrome.css inside a folder called chrome inside your profile folder. Add the following content:

.emailStar {
    display: none !important;

If you ever want to see it again just delete those lines.