Firefox 38.0.1 tools/options/content/colors won't display color options if it's already always using (say) white text and gray background.
Change the colors to "always use white text and gray background."
If you go back to the (new) tools/options/content/colors, the color choice is now not displayed.
This is called dead in the water.
Change the colors to "always use white text and gray background."
If you go back to the (new) tools/options/content/colors, the color choice is now not displayed.
This is called dead in the water.
Alla svar (2)
Ha ha, the setting is applied too thoroughly.
There is a workaround, which is to change the control that says "Always" to either "Never" or "On with High Contrast themes" (assuming you are not using such a theme), OK that, then return to the dialog and normal colors should be in effect.
Perhaps someone already has filed a bug for this but, if not, you can file a new one here:
You can use the NoSquint extension to set font size (text/page zoom) and text color on web pages.