Simple & compatible Rated 5 out of 5 stars

Very simple to use and yet, it is compatible with "Movable Firefox Button v1.4" - you don't have to disable the latter. Only do a mousey right-click on the Firefox-button and you'll see the color palette under the Firefox-button-menu. Push the Esc-button and the palette remains. Now choose your color (my beta-Fox-button is beta-blue).
And I fully agree with the review of "Linkz57": "...would like more options with this addon like removing the gradient and outline, or choosing a second color for Private Browsing Mode..." and a "RGB slider" for more colors would be nice.

Additional hint: After each re- or newstart of your Fox disable once the (restartless) "App Button Color" and then re-enable it, if you wants to call the color palette without the need of disabling the (restart-needing) "Movable Firefox Button".