Uploaded the example image directly in case the imgur link is not acceptable.

Hello? Mozilla staff?

You can use a custom style rule to remove the background. There may or may not be another way.

I use the Stylish extension to apply custom style rules to Firefox. However, it also is possible to create a userChrome.css file so that you do not need to add an extension.

Here's a rule I threw together but only gave light testing:

@namespace url(http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul);
/* Clear background and border/shadow from Navigation Toolbar */
#nav-bar {
    background-color:transparent !important;
    background-image:none !important;
    border-color:transparent !important;
    box-shadow:none !important;
}

The result is displayed in the attached screenshot. Apologies for any mismatch with your CTR, as I am not very familiar with its options.

Thank you very much, it worked really well.

I wish I had back all the time I've wasted trying to undo FIREFOX'S UPDATES that it now installs w/o my permission. THIS is why I resist updating because new and improved Firefox always is new and ANNOYING and NON-FUNCTIONAL.

Going from site to site trying this and that to FIX FIREFOX is so unnecessary if they would just LEAVE STUFF ALONE. Do their stupid security updates (if that's what they really are) BUT LEAVE EVERYTHING ELSE ALONE.