I have no idea about how relevant it is, but every few months I see a website which seem to be tested in Chrome only, because it blatantly don't load in Firefox or Safari. Like, at all, blank page. Or have some buttons like "Purchase" not working.
When you see this, complain and let them know. I've had some success complaining about this to medium sized companies and getting updates.
My retirement account and healthcare provider both have this problem. In one case I had to manually delete a disabled attribute on a button to get signed in because something broke halfway through a multi-step workflow.
Can't tell you my distress when government websites use Google captchas and on Firefox you're stuck and can't go through your business...
Google CAPTCHAs work in Firefox for me, on multiple devices and OS's.
Do you work at Google maybe? They don’t work for me either.
The Irish tax authority's login portal is completely broken in Firefox. It's the only website I need to have a separate Chromium install for.