The usual culprits are government, bank, or google websites. Youtube and Google docs occasionally break in amusing ways. Definitely Google's fault to a large extent, but it is what it is.

A friend recently got into making JS games, the FPS is always unplayable on Firefox. I'm not going to be snotty and ask someone doing this for fun to test stuff on multiple browsers.

SVG rendering occasionally slows down to a crawl. I used to open up bug reports ten or so years ago when I worked on something SVG related, but none of those bugs got looked at from what I can tell.

If I always have to have Chromium installed as a fallback, might as well use it.