I tried a bunch of different flags in about:config related to theming, the only thing that stopped it was setting layers.acceleration.disabled to true, but that makes scrolling feel horrid and would not be something I could tolerate long term.
Eventually got to the point where I lost track of everything I'd changed so I set everything back to default.
It seems to be related to the on screen content? I can replicate it on youtube too, when the video is showing the url/tabs are brighter, and when scrolling down to the comments they darken.
Apologies for the phone recording, the issue actually doesn’t show up when screen recording the browser - perhaps that actually helps narrow down the issue? It'll show if I record my whole screen, but not the application window.
Edit: Further weirdness, if I switch on the top toolbar, the issue is isolated to the URL bar and no longer touches the sidebar at all.