I've been noticing for a while that Youtube chapters were very hard to read if they were displaying over a white background. I thought this was just a Youtube issue but then I decided to boot up Edge and Brave to see if the issue still persisted. It doesn't. What happens in those browsers is that there is that the bottom part of the video is darkened just enough so that when the chapters are displayed there is enough contrast to read them. In Firefox this overlay cuts out before it reaches the chapters making them very difficult to read. I tried in Troubleshoot Mode and the issue persists.

I've linked pictures that demonstrate the difference.