From MozillaZine Knowledge Base

This article was written for Thunderbird but also applies to Mozilla Suite / SeaMonkey (though some menu sequences may differ).

By default, Thunderbird displays the signature text, and the signature separator ("-- ") above it, in the color gray. If you wish to change this to another color, insert the following into your "userContent.css" file.

/*
 * Change display color of text and links in signature
 */
.moz-txt-sig, .moz-signature {
  color: black !important;
}
.moz-txt-sig > a, .moz-signature > a {
  color: blue !important;
} 

The above example will make the signature text black and links in signatures blue. You can use other colors as desired.

Notes: 

• This changes only the way that signatures are displayed in Thunderbird and will not affect the color of any signatures that you use with outgoing messages. However, recipients who use Thunderbird normally see gray signatures unless they have changed the setting too. Other mail clients might do similar things.
• If you use an HTML signature file, it can specify its own color for the text but not for the signature separator ("-- ").

See also

• Signatures (Thunderbird)