ColorfulTabs for Firefox
To start sprucing up your tabs, just install ColorfulTabs into Firefox 3.0. Its palette of pleasant, low-key colors doesn't require much fine-tuning. However, the Options menu offers an assortment of tweaks: adding a background behind your tabs, adjusting the "fade" or transparency, or establishing color presets for certain preselected sites, and so forth.
My quibbles are few. I'd like the ability to establish color presets for my own favorite sites--not just for the ones in the short list. And if you're looking for a full-blown tab manager for Firefox 3.0, you might do better to wait for a stable version of Tab Mix Plus that supports 3.0. But if you just want to add a little beauty and distinctiveness to your browser tabs, ColorfulTabs is a good way to go.
Note: This link takes you to the vendor's site, where you can automatically install the file into your Firefox browser. This program is donationware. It is free to try, but the author accepts and encourages donations towards further development.