Probably because a browser like FF has the goal to load and display arbitrary dynamic content in realtime like a reddit infinite scroll with various 4k videos and ad bullshit, whereas the game has the goal to render a known, tested number of pre-downloaded assets in realtime.
Also on shitty pages the goal is different-- load a bunch of arbitrary adware programs and content that the user doesn't want, and only after that display the thing they want to read.
Also, you can click a link somewhere in your scrolling that opens a new, shitty page where you repeat the same crazy number of net connections, parsing ad bullshit, and incidentally rendering the text that the user wants to read.
If you want to compare fairly, imaging a game character entering a cave and immediately switching to a different character like Spiderman and inheriting all the physics and stuff from that newly loaded game. At that point the bulk of your gameplay is going to be loading new assets and you're back to the same responsiveness problems of the shitty web.
Edit: clarification