Firefox 4's share shot up 11% the first day after Mozilla started offering users the upgrade last week, and climbed 30% in four days.
The boost moved a long-time Mozilla employee to compare the gains of Firefox and Microsoft's Internet Explorer 9 (IE9) since the two browsers debuted last March.
"IE9 will never catch up to Firefox," said Asa Dotzler, Mozilla's community coordinator for Firefox marketing, on his personal blog. "It will be a year or two before [Microsoft] can move the bulk of their IE7 and IE8 users forward."
According to Irish Web metrics company StatCounter, Firefox 4 usage share jumped from 10.3% to 11.4% on the day after Mozilla began asking existing users if they wanted to upgrade to the new edition. The 1.1-point gain represented an 11% increase.
By Monday, Firefox 4 accounted for 13.2% of all browsers, a 30% increase from the previous Thursday.
IE9's progress has been slower: In the first four days after Microsoft turned on the Windows Update upgrade April 18, IE9's share climbed by 20%. Since then the browser has gradually grown its share, breaking the 4% mark for the first time last weekend.
Microsoft and Mozilla have been skirmishing over browser share since the former launched IE9 March 14 and the later released Firefox 4 on March 22.
After Mozilla boasted of the early download tallies of Firefox 4 -- 7.1 million the first day, 8.8 million the next day -- Microsoft argued that the comparisons between the browsers were "premature at best, and misleading at worst" because IE9 had yet to hit Windows Update.
Microsoft has said that all IE7 and IE8 users on Windows Vista and Windows 7 will have seen the upgrade offer by the end of June.
It shouldn't surprise anyone that Firefox 4 has a much larger share than IE9, even if Mozilla turned on its upgrade offer two weeks after Microsoft. Unlike IE9, which runs only on Windows Vista and Windows 7, Firefox also works on Windows XP, Mac and Linux.
Microsoft made the decision to leave Windows XP behind because it did not want to create a new browser for what one executive called the "lowest common denominator," the 10-year-old XP.
"[Supporting XP would have been] optimizing for the lowest common denominator. It's 10-years-old. That's not what developers need to move the Web forward," said Ryan Gavin, senior director of IE, in a March interview with Computerworld.
That same day, Dean Hachamovitch, who heads the IE group, said, "Other browsers dilute their engineering investments across systems" as he argued during IE9' launch event that Microsoft's browser was best because it ran only on Windows.
Mozilla has taken exception with that claim as well.
For all their individual gains, neither Firefox 4 or IE9 has managed to stem the overall decline of their makers. By StatCounter's data, both Firefox and Internet Explorer lost share in the first week of May compared to April's average.
All versions of Firefox accounted for 29.6% of the browsers used in the first seven days of May, compared to 29.7% during April. Meanwhile, IE's total share dropped from 44.5% in April to 43.8% in the first week of May.
Gregg Keizer covers Microsoft, security issues, Apple, Web browsers and general technology breaking news for Computerworld. Follow Gregg on Twitter at @gkeizer, or subscribe to Gregg's RSS feed . His e-mail address is firstname.lastname@example.org.