They also definitely advanced the CSS more than Netscape.

modern DOM, Ajax, modern CSS, bunch of other small details of JS, all iirc came from IE.

Development between IE4 and 5.5 was massive.

You and the grandparents are thinking of a slightly earlier time. Yes IE jumped over Netscape massively in technical terms. Then Netscape died an MS, having mission accomplished, basically stopped touching the browser.

The problem wasn’t per se that MS stopped (having a stable platform that doesn’t change continuously is in principle a good thing), it was most of all the many quirks, inconsistencies and bugs that IE had.

Even among browsers with the same version numbers! I recall array.push or array.pop missing on some Windows PCs with identical IE6 versions. It had to do with the upgrade path that the PC took.