The company also patched six Firefox vulnerabilities, and released a security update to the nearly-two-year-old Firefox 3.6 to quash a single bug there.
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Mozilla also tweaked Firefox's Interface on Mac OS X 10.7 to support Lion's two-fingered swipe gesture for navigating backward and forward through already-viewed pages or sites.
The Windows version did not spot any noticeable interface changes.
As part of the upgrade to Firefox 9, Mozilla also patched a half-dozen vulnerabilities, four of them rated "critical," the company's highest threat warning. The other flaws were rated "high" and "moderate."
The most serious of the six was actually a bucket of 23 memory bugs that developers found and fixed in the core browser engine.
"Some of these bugs showed evidence of memory corruption under certain circumstances, and we presume that with enough effort at least some of these could be exploited to run arbitrary code," Mozilla wrote in the accompanying 2011-53 security advisory.
Mozilla also released Firefox 3.6.25, the latest security update for the still-supported 2010 browser, re-patching a single flaw on Mac OS X that was originally -- and incorrectly -- addressed in late September.
Firefox 3.6 may be on its last legs: On Dec. 1, Mozilla offered those users a newer edition in an attempt to move them to the rapid-release schedule that produces an upgrade every six weeks.
It's unclear how successful that offer has been -- Mozilla hasn't released its own data -- but statistics from Irish metrics company StatCounter showed that Firefox 3.6's share has dropped by eight-tenths of a percentage point since the first of the month, more than during all of November.
Firefox still does not sport the long-promised "silent update" mechanism that will put it on par with Google's Chrome, which upgrades itself without any user interaction.
Silent updates, which have been on Mozilla's radar since the summer of 2010, have been again delayed, according to the company's website: The final piece of the service is now slated to appear in Firefox 12, currently set to release in April 2012.