Firefox's user share on all platforms -- desktop and mobile -- has plunged in the last two months as its desktop browser continued to bleed and its attempt to capture users on smartphones failed to move the needle, new data shows.

Apple's Safari fared almost as poorly since April, also losing significant user share, with a continued decline on mobile and a sudden slide on the desktop to blame.

During June, 17.3% of those who went online surfed the Web using a mobile browser, according to Aliso Viejo, Calif.-based Net Applications. Mobile browsing's climb of nearly 6 percentage points in the last 12 months represented a growth rate of 52%.

As in April, when Computerworld last analyzed desktop + mobile browser user share, June's numbers put the hurt on Mozilla most of all: Firefox's total user share -- the combination of desktop and mobile -- was 12.9% for June, its lowest level since Computerworld began tracking the metric five years ago, and 1.2 percentage points lower than just two months before.

Mozilla's problem remains an inability to attract a mobile audience. Although the company has long offered Firefox on Android and its Firefox OS has begun to appear on a limited number of smartphones, its mobile share was just seven-tenths of one percent, about three times smaller than the second-from-the-bottom mobile browser, Microsoft's Internet Explorer.

Firefox hasn't helped itself of late, either. For the eighth straight month, the desktop version lost user share in June, falling by 1.3 percentage points to end with 15.4%. In the last year, Firefox's desktop user share as measured by Net Applications has dropped 3.6 percentage points, representing a 19% decline.

The timing is terrible, as Mozilla's current contract with Google ends in November. That deal, which assigned Google's search engine as the default for most Firefox customers, has generated the bulk of Mozilla's revenue. In 2012, for example, the last year for which financial data was available, Google paid Mozilla an estimated $272 million, or 88% of all Mozilla income.

Going into this year's contract renewal talks, Mozilla will be bargaining from a much weaker position, down 43% in total user share since June 2011.

Apple remained behind Mozilla in desktop + mobile browser user share, with a cumulative 12.3%, down from 13.1% two months earlier. Nearly two-third of its total was credited to Safari on iOS.

But the browser was hit by a one-two punch in June: Safari on iOS continued to shed share in June -- it's dropped 7.8 percentage points of mobile-only share in just the last 90 days, a 14% decline -- and the desktop version fell by four-tenths of a percentage point. Even so, the gap between Firefox and Safari has narrowed in the last two months, with the latter, even as it lost share, making up ground on the former.

Google was again the main beneficiary of the losses suffered by Mozilla and Apple, adding to its lead over both in June, when it had a combined desktop + mobile user share of 22.6%, 1.5 percentage points higher than in April.

Together, the stock Android browser and the newer Chrome -- primarily, though not exclusively on Android -- accounted for 39.4% of all mobile browsers by Net Applications' count. With Safari's downward trend and Google's pair on the upswing, the latter could take first place from Apple on mobile as soon as October.

But it was Chrome on the desktop that powered Google's rise in combined user share for June. The browser added 1.6 percentage points to its desktop-only number, averaging 19.3% for the month. That was desktop Chrome's largest user share since May 2012.

Microsoft's Internet Explorer (IE) continued to dominate the desktop, where its user share was 58.4% for June -- the highest since February 2011 -- but on mobile, IE remained stuck in the low single digits: 2% for the month. That put IE's combined share at 48.4%, the same as in April.

Overall, IE remained in a much stronger position than Firefox, as Microsoft has managed to reverse a long-running decline in IE's desktop share. But Microsoft faces the same dilemma as Mozilla: Without tablet and smartphone traction, which will rely on Windows gaining ground on the platforms, IE will slowly lose influence as mobile gradually erodes the personal computer's decades-long position as the first choice for browsing.

Norwegian browser maker Opera Software held a combined user share of 2.2% in June, up from 1.8% in April. Its mobile browsers, Opera Mini and Opera Mobile, accounted for 62% of the total. A recent deal that places Opera's Android browser as the default on new Nokia-branded, Microsoft-sold smartphones powered by an Android variant may fuel further increases.

Net Applications measures browser usage on smartphones, tablets and personal computers by tabulating approximately 160 million unique visitors each month who browse to the sites it monitors for customers.

When desktop and mobile browser data are combined, Google has a solid lock on second place, with almost as much user share as Mozilla and Apple put together. (Data: Net Applications.)

Gregg Keizer covers Microsoft, security issues, Apple, Web browsers and general technology breaking news for Computerworld. Follow Gregg on Twitter at @gkeizer, on Google+ or subscribe to Gregg's RSS feed . His email address is gkeizer@computerworld.com.

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