Start Firefox in Safe Mode to check if one of the extensions (Firefox/Tools > Add-ons > Extensions) or if hardware acceleration is causing the problem (switch to the DEFAULT theme: Firefox/Tools > Add-ons).

• Do NOT click the Reset button on the Safe Mode start window or otherwise make changes.

• https://support.mozilla.org/kb/Safe+Mode
• https://support.mozilla.org/kb/Troubleshooting+extensions+and+themes

If you do not keep changes after a restart then see:

• http://kb.mozillazine.org/Preferences_not_saved

Thanks, none of those solutions helped, not even with a blank profile.

I downgraded to 19.0.2 and it works again.

I noticed a different behaviour, when I uncheck the box on 19.0.2, this line appears in prefs.js:

user_pref("browser.display.use_system_colors", false);

While checking it, the line disappears.

On 20.0 it works the opposite way, the line appears checking the box (with true value of course) and disappears unchecking it.

Do you have a user.js file in the Firefox profile folder?

• http://kb.mozillazine.org/Profile_folder_-_Firefox

Are their any high contrast setting on the OS level selected?

You can check for problems with preferences and try to rename or delete the prefs.js file and possible numbered prefs-##.js files and a possible user.js file to reset all prefs to the default values.

• http://kb.mozillazine.org/Preferences_not_saved
• http://kb.mozillazine.org/Resetting_preferences

Thanks for your help but it still doesn't work.

I thought user.js would solve the problem but it's like there's an upper level setting that overrides even user.js.

I repeat that I tried with a completely blank profile so everything as default.

Only via a mozilla.cfg file you can override specific pref settings or via an extension that makes changes in a very late stage (i.e. after all content has been loaded).

See:

• http://kb.mozillazine.org/Locking_preferences

These functions can be used in the mozilla.cfg file:

defaultPref();  // set new default value
pref();         // set pref, but allow changes in current session
lockPref();     // lock pref, disallow changes

Solved! thanks again for your help.

The problem was this file: /opt/firefox/all-gentoo.js

It contains some prefs that override everything (it's loaded like mozilla.cfg) so it's definitely related to Gentoo.

I'm posting just to add some more info in case it helps anyone.

I ran into this problem today too after changing to a high contrast theme (using Windows 7). It was happening on a fresh install of Firefox 21.0

My system background color was washing over many websites and graphics, and the text colors were sometimes replaced as well.

All I had to do to fix the problem was to press the following: [left ALT] + [left SHIFT] + [PRINT SCREEN] and then restart Firefox.

This toggles the windows High Contrast setting. This will only work if the ability to toggle is enabled in Control Panel > Ease of Access > Optimize Visual Display

The only minor issue that remains after that is text boxes (like the one I'm using to type this) still seem forced to use my system theme font color for "Message Box", although I don't mind. For comparison, my install of the Google Chrome browser has message boxes that don't do this.