Y'see, Firefox was able to clock up nearly 7 million downloads in the first day of availability, or almost triple the downloads that Microsoft was able to muster.
Presumably had Microsoft actually allowed Windows XP users to benefit from its latest and greatest window to the web, many more IE 9 downloads could have occurred, but with MS making the decision to keep XP out of IE 9's domain, as it were, we see the dramatically first day download difference.
Of course, it's not just how many downloads each browser gets on day one that counts, no - it's obviously a much longer game that the browser makers are playing, and whatever happens, both browsers will see tens of millions of users in relatively short order.
Firefox 4 promises to be faster, nicer looking, easier to use and be compatible with more of the latest web standards - like all new browsers on the market are often wont to do.
Given the increased security within new browsers, it's natural to recommend existing Firefox users consider upgrading to the newest version sooner rather than later, although the endless plug-in update game might see some plug-ins needs updating first.
If you aren't a Firefox user, then you might want to download and install it anyway so that you have at least one, if not more, alternate browsers on your computer.
Each browser is, after all, a slightly different window unto the webified world, and it's always handy to have a spare one to hand!
Firefox 4's download stats page shows well over 10m downloads thus far.
This is broken down as being 4.7m downloads in Europe, 3m in North America, 1.7m in Asia, 726k in South America, 200k in Africa and 150k+ in Oceania.
That means there's plenty more downloads worldwide still to come!