I attach screenshots, too. Images 1 and 2 (dark) are what I get, whereas 3 and 4 (white) are what I want to get.

Maybe check the default background color.

• Edit > Preferences > Content : Fonts & Colors > Colors

Nope, that's not the case, unfortunately. It only happens to Google pages, not to backgrounds in general. Thanks anyway.

Are you using code in userContent.css or extensions like Stylish?

It is possible that an Ubuntu extension is causing this.

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 > Appearance).

• 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

You were absolutely right there, safe mode did the trick all right! Then I confirmed that there was an extension called dark google or some rubbish like that, deleted it and restarted Firefox in normal mode --Google classic came back like a charm! Thank you very much, cor-el!

You're welcome.

If you use this as your default homepage you won't see the Google black bar.

https://www.google.com/?esrch=Agad::Public