This script is based on/inspired by LouCypher's Colorize Firefox 2.0 Active Tab; I just colorized the inactive tabs, too, with a complementary color. The sample screenshot is red-themed (Red metallic, more or less), but you can obviously choose any colors (I also have a Blue metallic). The active text is no longer bold and the focus ring is removed, plus I keep the hover color the same as the non-hover color on the active tab.
15Apr2008: reduced height of bottom border