Standards Compliant Browsers
Microsoft was very slow to support new standards in web technology when IE6 held a monopoly of browsers. Partly because they didn’t have to. Partly probably because IE6 was probably a complicated code base to work with.
This was frustrating for Firefox the open source web browser that was trying to become more relevant.
jQuery was very helpful in allowing web designers to support new standards and yet still work on IE6. jQuery basically has a whole bunch of work arounds to do the same things in IE6’s proprietary way that Firefox would do with modern standards.