In addition to what others have said, don't forget that ActiveX also played a strong role. ActiveX was pretty widely used in business-oriented web applications,

In addition to what others have said, don't forget that ActiveX also played a strong role. ActiveX was pretty widely used in business-oriented web applications, which hindered business adoption of firefox even among those companies or individuals who wanted to move to it. There was also a general distrust of open source software during the time that firefox was gaining adoption. With so much malware being spread via the browser (IE6), there was a fear that firefox's open source development model and perceived immaturity were a security and support risk.