It doesn't sounds like an intentional change. Perhaps the update created a new incompatibility with a theme or add-on, or with your graphics card/chipset driver software.

Could you test in Firefox's Safe Mode? In Safe Mode, Firefox temporarily deactivates extensions, hardware acceleration, and some other advanced features to help you assess whether these are causing the problem.

If Firefox is not running: Hold down the Shift key when starting Firefox.

If Firefox is running: You can restart Firefox in Safe Mode using either:

• "3-bar" menu button > "?" button > Restart with Add-ons Disabled
• Help menu > Restart with Add-ons Disabled

and OK the restart.

Both scenarios: A small dialog should appear. Click "Start in Safe Mode" (not Refresh).

Any improvement?

Yes, when I started in safe mode, my menus "came back" with black on white... but as I've not changed anything, except upgrading to latest Firefox... why the change and how can I fix this?

Ah... I removed "Ad Blocker" and that fixed the problem. I never would have thought that Ad Blocker (or was it UBlock? Anyway, that appears to have been the offender/screw-up add-on.

Thanks for the help!