I just ran into a somewhat important website that didn't work right in Firefox. I tried creating an online bank account with Discover. The register account process only worked with chromium. Once I had an account, though, I could use it through Firefox.

New gmail is pretty awful in FireFox.

Not mainstream sites, but I do seem to come across more sites that I have to open Chrome for.

I used to put it down to my plugins, but there are some sites (which of course I cannot name) that don't work, even with noscript / ublock disabled.