IE just made it so much easier to move fast in web development. People blast IE, but I really don't think the web would be as good as it is today without it.
Contrast that with what Chrome has done to the web. Not good.
Other browsers have done far more to get us to where we are.
Chrome introduced a modern security model and many useful APIs and protocols (including HTTP/2). Firefox popularized the concept of browser extensions and developer tools like Firebug. Safari made much possible by opening WebKit, such as Chrome and mobile browsers. Opera invented half of the original browser concepts including tabbed browsing.
AJAX was largely responsible for the initial push to modern, dynamic websites, but it doesn't score enough points to make Microsoft the good guy in this history.
Just to give credit where it is due: they had to, since they used KHTML, under LGPL, a pretty good browser engine at the time.
Anybody remember LAYER tags?
Prior to AJAX, sending info was done by loading a "transparent.gif?value1=x&value2=y&value3=z"
It's not technology that has ruined the web. It's greed.
AJAX made that cleaner and broadened its use, but yes, you could already make requests with Javascript that the user didn't want you to or didn't care to make for any purpose of their own.