Firefox support had an answer for me in less than 12 hours.
There is an option under tools whereby you can set text and background colors that will override HTML /CSS
on that dialog there is a checkbox "Allow pages to choose their own colors, instead of my selections above"
Tools > Options > Content : Fonts & Colors > Colors : [X]
Once I checked that the CSS colors and background image were displayed!
At some time in the distant past I must have turned that off in order to read one of those ghastly pages with unreadable color settings. Sigh!
Onward through the fog.
Bill