Google has a reputation for performing user interface experiments on its users. Typically this is more of an issue for logged-in users, but it can affect logged-out users from time to time as well.

If you see it again, try clearing cache and deleting Google cookies to see whether that removes you from the test group. Usually it does, but not always.

(1) Clear Firefox's Cache

See: How to clear the Firefox cache

If you have a large hard drive, this might take a few minutes.

(2) Remove the site's cookies (save any pending work first). While viewing a page on the site, click the lock icon at the left end of the address bar. After a moment, a "Clear Cookies and Site Data" button should appear at the bottom. Go ahead and click that.

In the dialog that opens, you will see one or more matches to the current address so you can remove the site's cookies individually without affecting other sites.

Then try searching again. Does that help?

Thanks for the reply. It has magically reverted back to normal now, before I had a chance to try your suggestions, but if it happens again, I will try what you say. Thanks for the help.

I think I remember having seen this issues reported a few years ago.

I see a Google.com button in the first screenshot that might indicate you visited a local Google site that still has this black background for the toolbar.