This AddOn can change the orange main menu button's color into any color you want, even transparent ones (color is used for light effect only if you reduce alpha to zero).
You can change the private browsing and the "normal" browsing color separately, both in ColorizedButton's Preference menu. Hint: This AddOn doesn't need a Firefox restart, but while testing I saw that the preference button in AddOn list is missing sometimes due to the restart thing. Dis- and Reenabeling caused it to appear again.
With the Australis update of Firefox (Firefox 29), this add-on is discontinued. If you know a good no-australis fork and would like me to support it, feel free to contact me, though.