Mozilla late Tuesday patched 13 bugs in Firefox, nearly half of them labeled "critical," as it closed support for the two-year-old Firefox 2.0 by releasing that version's final security update.

The update patched slightly more vulnerabilities in Firefox than the last two security updates in November and late September.

Firefox 3.0.5 fixes a total of 11 flaws, six rated "critical," one "high," one "moderate" and three "low" in Mozilla's four-step scoring system. Most of the critical bugs could be used by hackers to crash the browser, introduce their own malicious code into a vulnerable system or both.

Among the most serious were a trio of vulnerabilities in the browser's layout and JavaScript engines, while others included XML binding and session restore bugs that could let hackers conduct cross-site scripting attacks, which are often used in sophisticated identity theft schemes. Tuesday's fixes in the layout and JavaScript engines follow an identical number of patches applied to the same components last month.

The single vulnerability pegged as high also involves data theft, but how much information criminals might be able to steal was tough to predict, Mozilla said. "How much data could be at risk would depend on the format of the data and how the JavaScript parser attempts to interpret it," the advisory said. "For most files, the amount of data that can be recovered would be limited to the first word or two. Some data files might allow deeper probing with repeated loads."

Mozilla also updated the older Firefox 2.0 line to Version 2.0.0.19, patching 10 vulnerabilities in all, eight of them shared with 3.0.5. Of the total, only three were rated critical.

As per Mozilla's support policy, Tuesday's Firefox Version 2.0.0.19 was the final security update for the browser that debuted in October 2006. "Mozilla is not planning any further security and stability updates for Firefox 2, and recommends that you upgrade to Firefox 3 as soon as possible," said Samuel Sidler, a Mozilla engineer, in a post to the "mozilla.dev.planning" message group Tuesday. "It's free, and your settings and bookmarks will be preserved."

Although the older browser is now officially retired, users can, of course, continue to use it. However, as Sidler mentioned, Mozilla has urged users to upgrade to Firefox 3.0, which launched last June. Since then, it has twice offered what it dubs a "major update" to users of the older browser, hoping to get them to move up. The most recent upgrade offer went out two weeks ago. Mozilla plans to make one final offer sometime early next month.