"Similar slow uptake was seen after 10.0.1 chem spill was unthrottled on February 12," said the company, referring to a rapid-response patch that also addressed a crash bug.
Mozilla had a theory about why chem spill updates slowed down upgrading.
"After a chem spill, users still on previous major [version] must download a full update (approximately 20MB) instead of partial [update] (approximately 8MB to 10MB)," the release engineering group said.
In that scenario, come of the users who had not opted for silent updates skipped the upgrade to Firefox 13 because of its larger download size.
The team also posed several other possible explanations, including the season of the year and the fact that the upgrade process is focused first on getting users to Firefox 12 because it is the final edition that supports Windows 2000 and older versions of Windows XP.
Other factors may have also contributed to the poor uptake of Firefox 13.
Reaction to Firefox's "new tab" page and the revamp of the browser's Home page -- both debuted in Firefox 13 -- have been largely negative, Mozilla has admitted.
During a meeting on June 13, Mozilla noted that three-fourths of all feedback on the new tabs page was negative, with just 10 percent approving the changes. Response to the revised Home pages, said Mozilla, was also "generally unfavorable."
It's possible that some users have switched off automatic updates to prevent Firefox from upgrading to version 13 and its new tab and Home pages.
But StatCounter's data points to the June 15 emergency update as the most likely culprit. While Firefox 12's and Firefox 13's adoption was virtually identical through the first nine days of availability, their uptake lines departed dramatically on day 10, with Firefox 13 falling increasingly behind Firefox 12's performance.
Day 10 was June 15, the date Mozilla started shipping the chem spill Firefox 13.0.1.
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 email@example.com.
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