I think that's the worst part of the new Firefox UI. Firefox got rid of the defined, rectangular tabs using a colored background with lines between them. Now it's white rounded rectangles floating in a light-gray background with no other visible distinction. Even with perfect eyes, it was hard to tell what was going on without taking a moment to really focus on it.
It clashes heavily with the container tabs add on as well. The new UI adds a coloured bar on top of a container tab, which I always mistake as an open tab since it's far more visible than the actual open tab indicator
There are some tweaks out there which patch the CSS to bring stuff slightly closer to the original style.
Oh, I've managed to get it looking basically exactly like Firefox 90 again for me, but it's still a hassle to have to inject custom CSS on every machine to get an easily-usable interface. And I feel bad for people like my elderly dad who don't have that know-how and will have to just live with Mozilla's indifference to accessibility.
"Modern" UIs most often are anything but clear and less noisy. Part of what made the Windows 95 design so strong was that UI elements were separated by visible lines, you exactly knew what the touchbox of any element was, the behavior and look of UI elements, menus and icons were generally consistent across applications...