Also compatibility, marketing, and being installed out of the box.
In its heyday, Firefox had between 25% and 50% market share, depending on country. Compared to Internet Explorer 6, it was:
* More stable.
* Less buggy.
* More secure.
* More featureful.
* Usually faster than IE6, though it took a bit longer to start.
* Better development tools, which indirectly lead to web pages working better in Firefox than in IE6 even if that's not what management wanted.
* ActiveX, which is clearly the way Microsoft wanted you to make "web apps," had to be hidden behind scare prompts, so public web pages couldn't use it.
Here's Internet Explorer's big selling points, which allowed it to hold onto between 50 and 75% of the market share:
* It shipped with Windows.
Google Chrome only managed to unseat it by paying OEMs, which means that, de facto, Chrome now ships out of the box with Windows, too.