* It considered width to include border and padding, when the spec and other browsers did not: https://www.jefftk.com/p/the-revenge-of-the-ie-box-model
* Floated block elements would get their margins doubled, so people would typically use padding instead.
* Its implementation of height was more like min-height.
While I'm sure Firefox and Chrome have open bugs that are older than that, I'd expect them to be much less important than these critical misinterpretations of CSS.
(To give a sense a sense of the scale I think we're talking about, this would be a bug that, say, more than half of web developers have to understand and work around in a typical month)
Youtube using polymer/shadowdomv0 and absolutely destroying performance on Firefox is a an easy example. Google Earth used non-standard APIs.
Otherwise, have a quick search for "non-standard" on MDN, and see how many of those Chrome implements.