Do yourself and the community a favor and when you see a website that doesn't work well make the Firefox Team aware of it[1].
They have a dedicated Team to check those issues.
I use Firefox in excess of 10 hours every day and don’t have any problems so I find this statement very surprising. I’m not sure about Chrome being the de facto standard either and even if it is, it doesn’t mean you have to follow it.
Such as?
Notion (can't scroll down with the keyboard, only the mouse) or Intercom (customer's replies are not displayed until you reload).