One thing that sort of bugs me about Firefox is there is no real way to tell the difference between your active tab and your non-active tabs, other than a slight change in the background color. The User Style, FF3+ Increase active tab minimum width, will allow you to increase the minimum width of the active tab to make it stand out a little more.
To get started, make sure you have the Stylish extension installed for Firefox, and then head on over to the userstyles.org website. You can choose from these four sizes when installing.
Once you select the minimum width you want your active tab to be (for reference, the default size is 100px) hit the button labeled, "Install with Stylish" and follow the on screen instructions.
Sometimes it is the little tweaks that are the most impressive. You can pick up FF3+ Increase active tab minimum width from the userstyles.org website.