In previous blog posts, John Lilly has explained that we cannot identify precisely how many users Firefox has because there is nothing uniquely identifying in the Firefox browser and that's something we're not interested in changing just for the sake of better accounting.
Because of that, we can only estimate. The best method we've come up with is to count the number of security "pings" we get in one day. (Firefox pings a Mozilla server for security changes about once per day) and multiply that active daily users number by three to get active monthly users. (See John's blog post for more on this.)
Over the last couple of weeks, we averaged about 90 million of these pings per day. Multiplied by three, that gives us an active user base of approximately 270 million.
I think that number is probably a bit conservative but it's very likely in the ballpark. (See my earlier post What Does the Web Look Like for some broader estimates.)
We could get an even better number and make this all a bit less opaque and confusing if the other major browser vendors, Microsoft and Apple, would release some of their numbers and estimates. I can't see what value there is in keeping such things a secret so maybe we'll get lucky and they'll open up some.
Here's the graphic that some of you have been asking for:
And yes, the active daily users (ADU) number is an average. Yesterday we had over 98 million ADUs but, as I've discussed in a previous post, total Firefox and total Web users are highest on weekdays but drop sharply on weekends.
It really would be great if Apple and Microsoft would disclose more of their metrics. We'd be able to understand how the Web trends so much better. Until then, at least there's Mozilla metrics :-)