Some times the simple solutions, make the biggest difference to a user experience....

Colorful Tabs | Firefox Add-ons | Mozilla Corporation
The most beautiful yet the simplest add-on that makes a strong colorful appeal. Colors every tab in a different color and makes them easy to distinguish while beautifying the overall appearance of the interface. An essential.

UPDATE: It seems that the Colorful Tabs authors homepage is down, as a result I appear to be getting hits from google and comments about features. I'd like to stress that I have nothing to do with the development of Colorful Tabs and you have my apologies if my post appeared that way. You have stumbled across a post from my firefox recommendations category where by I post links to firefox add-ons I like have installed and recommend to my friends.