Think a bug report might get more attention. But I agree this is an obviously desired result, and they probably implemented the first Firefox version very simply. And yes, I've seen this too (not compared in Chrome or Edge though).
I'm guessing the main web page thinks it is displaying in the original area/size, and is adaptively picking a resolution that's good enough but no bigger/better. To see a better image while bigger you'd need to get more information.
Chrome might change the original request to get a better version (automatically change an attribute in the video object to match the new bigger displayed area?). While Firefox is honoring the original code, for better or worse.
Your result might depend where the video is from too. With Google being the owner of both Chrome and YouTube they likely work with each other to make things look great. And maybe even break competitor's results on purpose, if easy.
