1 billion downloads is pretty amazing when you consider that the overwhelming majority of those downloads resulted from word of mouth. Mozilla's advocacy community is unmatched in the software world.
Now, of course that doesn't translate into 1 billion users. Some of those downloads were second or even third downloads onto the same machine (though there were also many large-scale deployments at huge companies like IBM where a sysadmin made one download and pushed it out to tens of thousands of desktops.) But we're not talking about automatic updates either. These are real user-initiated downloads and that's a lot of pro-Firefox activity no matter how you slice it.
A couple of other measures that are relevant: Firefox's global usage share is somewhere between 22% and 28% depending on who you believe, and the Firefox active user base is north of 300 million users.
