It was a few years ago and I'm unable to reproduce it. Circles and circle approximations from straight lines seem to coincide exactly in Firefox and Edge now.

It was a few years ago and I'm unable to reproduce it. Circles and circle approximations from straight lines seem to coincide exactly in Firefox and Edge now. I was generating diagrams using my own code so I'm positive they were circular arcs. Tangents weren't tangent and it took me a while to realise that the problem was the browser rendering something that wasn't quite a circle. I assumed it was a bezier approximation but I didn't verify that. Whatever it was it seems to be fixed.