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I work in web and app development company and we don't check Firefox anymore, because it's the only outlier and has not many users. But mainly because we wouldn't have to do it for any other browser specifically and Firefox is not special in any way. The errors come from it being more strict, which might sound good, but it's actually really just inconvenient. The errors go from image alignment issues to apps not working at all. We don't fix any of that.