Doesn't matter. A monopoly is not when everyone chooses the same thing, but when everyone has to choose the same thing, because there is no (and can't be any) alternative. If you have a big share of the market, you just have a big market share. That's not the same as having a monopoly.
Monopoly in this instance would be a shorthand word for a similar situation. Yeah, there were alternative browsers you could choose from, but IE had such overwhelming control, they didn't really matter. It came packaged standard with Windows, and the vast majority of people used it rather than tracking down an alternative. For all intents and purposes, it was the only real player in the game for a good long while.
This isn't necessarily a bad thing when taken in isolation. Being the most popular or the default option isn't illegal, nor is it always harmful to the marketplace. The problem was that IE6 flat out
blew when compared to the competition, and was barely updated in the nearly 10 years it reigned supreme. It was an insecure, barely functional mess that hung around far longer than it should've. And since it came standard with Windows, nothing else had a real chance to grow in its place. Not until, like you said, Firefox started taking off and spreading by word of mouth. And it took nearly 4 years of steady updates before it even made a 10% dent in IE's userbase.
It is not Microsofts (or anyone's) obligation to make web developers happy.
You're right, it's not their obligation, but it certainly helps if you want to keep people using your platform. The problem with IE6 is that MS didn't have to even consider it, and they literally held back an entire tech sector for nearly a decade. It'd be like all of stuck using Windows 95 despite computer hardware coming along in leaps and bounds since it's release, because MS had such a stranglehold on the market they didn't have to worry about updating. There'd be so much more people could do with their computers, but...well...everyone's still using Windows 95, so they can't.
Hell, it created a mess so big, it even took MS years to get out from under it completely.
So did MS succeed in creating a browser monopoly? Not if you go by the bare definition thereof, no. But they did create a situation fairly similar to.