Not who you are replying to, but before I switched to Vivaldi (a Chromium fork), I saw lots.
Among them: Logging into some of my financial accounts doesn't work on Firefox. Enterprise software and gear like VMware and management UIs of various devices on the network. (They foolishly hard-coded their devices to reject any UserAgent strings that weren't Chrome, IE, or Edge.) Sites that use some kind of poorly-implemented tracking/fingerprinting to make sure you're a human. (I would routinely get stuck in infinite CAPCHA loops even on normal sites.) For a while, Slack video/audio calls did not work on Firefox because Slack chose to use codecs that FF didn't support. Video calls on FF are still hit-and-miss on various platforms, ran into it on Facebook just the other day.
These are all just off the top of my head, of course. There are plenty more that I've forgotten.
Among them: Logging into some of my financial accounts doesn't work on Firefox. Enterprise software and gear like VMware and management UIs of various devices on the network. (They foolishly hard-coded their devices to reject any UserAgent strings that weren't Chrome, IE, or Edge.) Sites that use some kind of poorly-implemented tracking/fingerprinting to make sure you're a human. (I would routinely get stuck in infinite CAPCHA loops even on normal sites.) For a while, Slack video/audio calls did not work on Firefox because Slack chose to use codecs that FF didn't support. Video calls on FF are still hit-and-miss on various platforms, ran into it on Facebook just the other day.
These are all just off the top of my head, of course. There are plenty more that I've forgotten.