A True Review posted a guide on customizing Firefox, here's a snip:
One of the best software advancements of 2005 has to be Mozilla's Firefox.
Firefox has gained a lot of notice this year; therefore, it is rapidly
gaining share on Microsoft's Internet Explorer. It's not only security that
fuels Firefox's gain, but also the ability to customize the browser to your
liking and extensions that streamline your browser experience. This article
looks at Themes, Extensions, active bookmarks, and changing the browser's
interface with Cascading Style Sheets. Read more here