I'm aware of a few things, myself:

1) Google properties

1a) Chromecast

2) a few web-based games that were really pushing the envelope on web APIs and didn't bother testing on Firefox

3) WebUSB, commonly used for some things like keyboard customization apps

Which Google properties are Chrome only? I'm not doubting you but the major ones (search, mail, maps, ads) are extremely cross-platform.

	▲	JoshTriplett 2 minutes ago \| parent [-]
		In the past there were features that didn't work at all; I used to hit those regularly. Device setup flows, AV features, etc. These days, it's never "this doesn't work on other browsers". It's always "this is worse on other browsers", whether because they don't test it or because they don't care.