I don't think stronger substitutes (Job's better products) are the correct answer - and as you observed, FF has only scratched IE dominance, not eliminated it.

I don't think stronger substitutes (Job's better products) are the correct answer - and as you observed, FF has only scratched IE dominance, not eliminated it. Better product certainlly have some relevancy, because improving IE (IE7/8) will make it good enough that most people will not need to change, but we will never really know how low IE6's marketshare could go if Microsoft had not improved it - maybe it would still be good enough for most users. At least, Windows was good enough for most users, even with better alternatives along the history - Next, BeOS, OS/2...