Two-thirds of the vulnerabilities patched were rated 'critical', the threat ranking that represents bugs hackers could use to hijack a system or infect it with malware. Of the remaining vulnerabilities, two were labelled 'high' and one each was judged 'moderate' and 'low'.
Among the flaws was Firefox's second 'binary planting' vulnerability. Some have dubbed the problem 'DLL load hijacking'.
Regardless of the term, the flaw existed in Windows applications that do not call DLLs (dynamic linked libraries) or executable files using a full path name. Instead, they rely on the filename alone.
The latter can be exploited by attackers, who can trick the program into loading a malicious file with the same title as a required DLL or executable. If attackers can con users into visiting malicious websites or remote shared folders, or get them to plug in a USB drive, they can compromise a computer and infect it with malware.
Binary planting or DLL hijacking attacks aren't new: The first evidence goes back a decade, and Microsoft was informed of the problem in August 2009 by researchers at the University of California at Davis.
They have also been used in the real world: the Stuxnet worm, which targeted industrial control systems and infected thousands of PCs in Iran, used binary planting techniques among its bag of tricks.
Mozilla patched the DLL part of the flaw last month when it updated Firefox to version 3.6.9. Tuesday's fix ensures that Firefox can't be used to launch rogue executable files. (An associated patch fixed the Linux 's version library loading bug.)
Firefox rivals Safari and Opera have received similar patches to plug their DLL load hijacking holes, but Google 's Chrome has not. One of the companies that raised the alarm last summer, Croatia-based Acros Security, has said users running Microsoft Internet Explorer (IE) on Windows XP are the most vulnerable to such attacks.
Other patches included in the update closed a trio of browser engine memory bugs, a cross-site scripting vulnerability and an issue with SSL (secure socket layer) certificates.
Among the other 40 non-security bugs fixed by Mozilla was one with the 'out of process plug-ins' (OOPP) feature that's designed to keep the browser up and running if Adobe's Flash Player, Apple 's QuickTime or Microsoft's Silverlight plug-ins crash.
OOPP debuted in Firefox 3.6.4 , which Mozilla launched last June.
According to Bugzilla, Mozilla's bug- and change-tracking database, the OOPP flaw reported Flash as crashing when the browser tried to play content on Turner Broadcasting System's (TBS) website. However, when OOPP was disabled, Firefox rendered the Flash content without problems.
Mozilla also updated the older Firefox 3.5, which is on borrowed time. Mozilla's website states that it will maintain 3.5 "with security and stability updates until August 2010", or two months ago. Typically, Mozilla ships patches for an older edition only six months after the release of a new version; Firefox 3.6 launched in January 2010.
Firefox 3.5 accounted for about 14 percent of all copies of Mozilla's browser used last month, said metric company Net Applications.
Users can update to Firefox 3.6.11 by downloading the new edition or by selecting 'Check for Updates' from the Help menu in the browser. Firefox 3.5 users can obtain version 3.5.14 by calling up the integrated update tool.
See also: Trojan forces Firefox to save passwords