Last month, it looked like Firefox's relinquishing of the second place browser spot was inevitable. In May, Mozilla's browser market share had dropped below 20 percent. It was just 0.14 points ahead of Google's Chrome.

But Firefox has somehow fought back. Chrome's share is down, Firefox's is up, and there's almost a point separating the two.

Internet Explorer dropped slightly while remaining dominantly in first, down 0.05 points to 54.00 percent. Firefox gained 0.35 points to 20.06 percent while Chrome fell 0.50 points to 19.08 percent. Safari and Opera both made small gains, of 0.11 and 0.03 points, giving them shares of 4.73 and 1.60 percent respectively.

The last couple of months have seen only slight declines for Internet Explorer. Microsoft's browser is still losing share, but at a much slower rate than we saw a year ago. This suggests the steady half a point per month erosion of its share may be at an end.

Firefox's gains and Chrome's losses are both a little surprising. Chrome suffered setbacks earlier in the year after Google penalized itself for improper advertising and promotion, but it looked like the company had resumed its trend of steady gains.

Counting browser market share remains difficult. The information source we use, Net Marketshare, strives to assess the proportion of Web users using each browser. To do this, it uses demographic data from the CIA World Factbook to provide national weightings that it applies to the raw counts it collects. Other sources, such as the widely quoted StatCounter, only denote the relative number of webpages viewed in each browser. That site explicitly eschews such weighting techniques. The main result of this is that Net Marketshare places greater importance on Internet Explorer-dominated markets such as China while StatCounter, in contrast, places much more weight on North American usage patterns. Accordingly, that metric gives Chrome a slight edge over Internet Explorer, at 32.76 percent to 32.31 percent.

Further confounding matters, Google announced at its I/O conference last week that it had 310 million Chrome users, and that there were 2.3 billion Internet users in total. Assuming that most of these Internet users use the Web at least occasionally, this gives Chrome a share of just 13.4 percent of the Internet-using public.

As ever, Safari is well out in front when it comes to mobile browsing. Android seems to have cemented itself as the number two mobile browser for now.

Chrome retains the distinction of being the only browser with an effective update system. While a few Chrome users do stick with older versions of the browser for one reason or another, they make up only 9.5 percent of Chrome's userbase. The vast majority of Chrome users are either on the current stable build or a beta of an upcoming version.

Firefox, in contrast, has left a lot of users behind. Even optimistically assuming that every user of Firefox 10 is on the (secure, patched) Extended Support Release version (rather than the insecure, unpatched mainline version), a quarter of Firefox users are behind the times, using flawed, exploitable software.

The situation for Internet Explorer is harder to gauge. First the good news; Internet Explorer 9 is continuing to make large gains. That is up almost a full point from May. Internet Explorer 6 and 7 continue to drop. Less encouraging is that Internet Explorer 8 is still the most widely used browser in the world, with just over one in four Internet users picking it for their browsing.

What we can't tell so easily is whether these users are patched and up to date. Internet Explorer 6, 7, 8, and 9 are all supported long-term and will all receive security patches until at least 2014 (for Internet Explorer 6) and as long as 2020 (for Internet Explorer 9). So potentially, all Internet Explorer users of these versions could be protected against most known flaws.

That's not likely, however. Many of those users could be using flawed, unpatched versions. We just can't easily tell. Microsoft doesn't change the version number whenever it fixes security flaws in Internet Explorer 6, 7, or 8. Internet Explorer 9 does have an "update version"—currently 9.0.7—but this isn't reported as part of the User Agent that is widely used to identify browser versions.

As such, the patch situation for Internet Explorer may be even worse than that of Firefox.

Chrome remains thoroughly dominant here at Ars, further increasing its lead over everything else.