Color Active Tab од fstephens

Perfect for users who navigate many open tabs.
Color choosers to select background and font color for the active tab.
Can apply a classic Teal theme, a night mode of soft black & white or restore your browser's native system default theme.
You can choose a user interface color to compliment the active tab color. The history drop-down menu will save the last five colors chosen, and dynamically preview them on holding the mouse cursor over a choice. Clicking a choice will apply it.
So, you can create a simple custom theme on the fly.
NOTE: Due to the all or nothing nature of theming in Firefox, it is not possible to just color the active tab and retain the rest of a custom theme.