Start Firefox in Safe Mode {web Link} by holding down the <Shift>
(Mac=Options) key, and then starting Firefox. Is the problem still there?

I tried holding down the Shift key (BTW, it's not Option on the Mac) and starting Firefox 42 in Safe Mode. Nothing changed regarding colors in preferences.

I bit the bullet and upgraded Firefox to version 42 on my MBA Mac and the color pref does work there so its probably a startup item or system preference conflict on my Mac Pro using Yosemite that is causing the problem. Next time I reboot I'll trying disabling extensions and see how that affects Firefox.

Shift versus option

I was wrong, option is the key to hold down for Safe Mode on OSX. But it still didn't work - that is I didn't get the Safe Mode dialog box and the colors were still missing. I think I'll try reinstalling Firefox...

This will happen if you override the website colors and have possibly enabled a High Contrast theme.

Make sure that you allow pages to choose their own colors.

• Firefox > Preferences > Content : Fonts & Colors > Colors : "Override the colors specified by the page with my selections above"

Try "Never" if the default "Only with High Contrast themes" isn't working.

I tried the "Never" option but the problem persisted.

I also reinstalled Firefox, tried different versions, invoked Safe Mode, etc. Nada.

I used the Command-Option-P-R trick to rebuild the desktop, used a cache cleaner, rebooted the Mac Pro with Shift key held down (essentially safe mode for the whole computer) but after all that I still couldn't get it to work. One last restart and still nothing. I shut down the computer last night and rebooted it this morning and voila, Firefox colors are back like nothing happened.

Go figure.

That was very good work. Well Done. Please flag your last post as Solved Problem so others will know.

I was about to mark this solved until I went to double-check my Prefs again after another restart. Firefox's content prefs had gone back back to all white.

This morning, after another restart, its back to normal colors.

I can't tell if it's my machine or Firefox without a lot of testing I don't have time to do.

There is no rush. Take your time.

https://support.mozilla.org/en-US/kb/how-to-fix-preferences-wont-save

Note: Some software, like Advanced SystemCare with Surfing Protection, can protect files in the Firefox profile folder against changes. If you have such software then check the settings or uninstall this software.

See also:

• http://support.apple.com/kb/HT3488?viewlocale=en_US

Option-Control-Command-8 : Invert/revert the screen colors