Where are the sites that don't work in Firefox?

Yeah we'd all like that. But for those people saying they wish the Foundation would prioritise it, it'd be great to hear some good ideas on how they could realistically promote Firefox against the juggernaut of Google pushing Chrome on google.com Microsoft pushing Edge in Windows.

The browsers are roughly feature-comparable, and people seem to want to log in to to sync their bookmarks etc. It's really hard to write a killer browser feature that only Mozilla could do and which Google could not quickly replicate in Chrome to keep their market share.

I feel the only things that Google could not/would not add to Chrome are features related to freedom (which 95% of the population doesn't care about and wouldn't switch to Firefox for), or something to do with ad-blocking, which is already done by extensions.