Earlier quoted context omitted.
Firefox out of the box indeed does not cause broken websites. However, the demographic of this forum probably will use Firefox with "Multi-Account Containers", "Temporary Containers", "uBlock Origin", and a few more. These are amazing for privacy and productivity, but will occasionally cause broken websites. Source: I am a Firefox-first user who occasionally uses plugin-less Chrome because the aforementioned plugins…
I have had sites that are definitely broken in Firefox even after a stock install with every last toggle/extension/script blocker turned off. It's fairly rare, but there are a slowly growing list of sites that don't behave properly under Firefox but work in Chromium.
So I'm curious, which sites are broken in vanilla FF?