How do you separate which of these are bugs in Firefox and which ones are bugs in the sites that don't test against more than one browser engine? Reminds me of

How do you separate which of these are bugs in Firefox and which ones are bugs in the sites that don't test against more than one browser engine? Reminds me of back in the day with the "works best in Internet Explorer" banners. The MKV thing sounds like a Firefox or codec issue, but I couldn't say. I haven't had issues with MKV or WebM, which is effectively a MKV profile, in many years.