My name is Philip, I’m a Senior Visual Designer working on Add-ons. I reached out to one of our designers who helped build Photon (our new visual design language for Firefox) and here’s what he said:
“There is a translucent overlay on background images to help keep some distinction between foreground and background. The problem is that we can’t know intent with themes because it’s just a background image.”
One of our designers is working on a theming extension that will allow more fine-grained control of how your browser looks (watch for that to be launched in Test Pilot in the coming months), but for now the active tab will have that overlay.
Thanks for reaching out,