A lot of it is lingering hatred over the situation at the turn of the millennium, where Microsoft was content to sit on their market dominance for several years, knowing most users weren't savvy enough to switch to another browser. Internet Explorer 6 was a barely-functional mess riddled with security holes, and it was out for
five years before IE7 arrived. It really held back the evolution of the web. No use targetting new standards if the most popular browser doesn't support them (hell, it barely supported then-current standards; go look at the source to a popular site some time, and see the length of the compatibility code for Internet Explorer 6).
After the rise of Firefox (and when Microsoft realized they were hemorrhaging market share), they stepped up their game and vastly improved their browser. It's now (relatively) secure, bug-free, and standards-compliant. It's even reached the point where I wouldn't recommend users immediately uninstall it in favor of another browser.