Firefox is ridden with decade-old compliance bugs, and is generally a pain to write applications and documents for. That's just part of why web applications and documents work worse in Firefox than they do in Chromium.
Mozilla should derive from Chromium, if they want to actually deliver a web browser worth choosing. They can even fight their small battles with niche concerns more efficiently this way; but alas they will choose to make their users suffer needlessly for the scintilla of a principle of “implementation diversity”, which in practice just means “Firefox workarounds”.