From MozillaZine Knowledge Base

To change the color of links in messages that you view, insert the following code into your "user.js" file:

//Color of links in messages
user_pref("browser.anchor_color", "#000000");

In this example, #000000 is for black. For other colors, you can try this Color Blender.

To turn off the underlining of links in messages, insert the following into your "user.js" file:

//Disable underlining of links
user_pref("browser.underline_anchors", false);