Mozilla on Wednesday patched 11 vulnerabilities in Firefox 3.0 -- and 12 bugs in the older Firefox 2.0 -- that could be used to compromise computers and steal information.

Wednesday's update patched virtually the same number of vulnerabilities as the last security upgrade seven weeks ago.

Firefox 3.0.4, the fourth update since Mozilla launched the browser in June, fixes six flaws marked "critical," two "high," two "moderate," and one "low" in Mozilla's four-step scoring system. Most of the critical bugs could be used by hackers to introduce their own malicious code into a vulnerable system.

Among the most serious were a trio of vulnerabilities in the browser's layout and JavaScript engines, while others included a buffer overflow bug in the HTTP index format parser and another flaw -- pegged as moderate -- in the file: protocol handler. Mozilla repeatedly patched protocol handler bugs in Firefox starting in July 2007.

That vulnerability was judged moderate by Mozilla because of extenuating circumstances. "It requires an attacker to have malicious code saved locally, then have a user open a chrome: document or privileged about: URI, and then open the malicious file in the same privileged tab," Mozilla said in its advisory .

Mozilla also updated the nearly-retired Firefox to 2.0.0.18, patching all but two of the same vulnerabilities fixed in 3.0.4, and several others for good measure. Of the dozen bugs, six were rated critical. The 2.0.0.18 update will be the next-to-last for the older Firefox 2.0, which will be dropped from support next month.

Before that happens, Mozilla will make one last effort to convince Firefox 2.0 users to upgrade. In two to three weeks users will again be prompted to upgrade in a repeat of an offer first extended in August. Mozilla has been very successful in convincing users to upgrade; as of the end of October, 73 percent of users were running the newer Firefox 3.0, reported Web metrics firm Net Applications.

Not updated on Wednesday was Thunderbird, which remains at version 2.0.0.17. It's not unusual for the e-mail client to lag behind Firefox in patching vulnerabilities; as in the past, several of the issues in Firefox are also present in Thunderbird. Because the most dangerous of the six shared vulnerabilities are in various elements of the browser's JavaScript support, Thunderbird users can protect themselves in the interim by disabling JavaScript.