Firefox is somewhere around fifteen percent, depending on whose figures you trust and which sites they measured. That number gets quoted as the story, and it is the less interesting one.
The real achievement is that Internet Explorer got a new version at all.
Consider the gap. Internet Explorer 6 shipped in 2001. The next release arrived at the end of last year. Five years, during which the web changed more than in any comparable period, and the browser used by the overwhelming majority of people on earth received security patches and nothing else.
That was not neglect or incompetence. It was a decision, and from where Microsoft sat it was defensible. Netscape was finished. The browser war was over, the browser had been established as a component of the operating system, and there was no remaining commercial reason to spend engineering money on it. So the team was largely dispersed onto other work, which is what any company does with a product that has won.
Five years of nothing followed, and every developer who built anything for the web in that period paid for it. Tabs did not exist. Standards support froze at whatever 2001 thought was correct, so a decade of specification work landed on a browser that could not use it. The rendering bugs became so well documented that working around them was a recognised professional skill.
Then a free browser from a non-profit started taking share, and development resumed within about a year of it becoming visible.
So the lesson is not that people prefer Firefox, though many do. It is that a monopoly product stops improving the moment the monopoly is secure, and resumes the moment it is threatened, and the interval between those two events is paid for entirely by users who have no way to register a complaint that anyone acts on.
Which is why the share figure matters more than its size suggests. Fifteen percent is not enough to win anything. It is enough to make the incumbent keep spending, and that spending is worth vastly more to everyone than which browser ends up in front.
My worry is what happens if the pressure comes off. If Firefox plateaus and stays comfortable, there is nothing structural preventing exactly the same five year silence from happening again, and no reason to expect a different outcome.