Mozilla Corp. has patched a pair of critical vulnerabilities in Firefox, taking the unusual step of updating the older version 2.0 on Tuesday but delaying the fixes for the newer version 3.0 until Wednesday.

Both updates, labeled Firefox 2.0.0.16 and Firefox 3.0.1, plug two holes rated "critical" by Mozilla, which uses a four-step threat ranking system. Firefox 2.0.0.16 was posted to Mozilla's servers Tuesday afternoon.

Firefox 3.0.1, the first update since the open-source browser was upgraded almost a month ago, won't reach users until Wednesday at the earliest, according to notes from a Mozilla status meeting published online.

One of the flaws patched in 2.0.0.16 and 3.0.1 was credited to security researcher Billy Rios, who wrote last month about a "blended" threat to Windows users who had both Apple's Safari browser and Firefox installed on the same system. Then, Rios said that Safari's "carpet bomb" bug -- first disclosed in May and patched in June by Apple -- could be combined with other vulnerabilities to attack not only systems with Microsoft Corp.'s Internet Explorer, but also those equipped with Firefox.

Back in June, Rios refrained from providing technical details or exploit code, saying that he had reported the vulnerability to Mozilla. "I'm sure we???ll see a fix soon," Rios said in a June 20 entry to his blog.

Mozilla confirmed Rios' findings. "Rios demonstrated that the so-called'Safari Carpet-bombing vulnerability' could be used for this, as well as other techniques that do not rely on that now-fixed Safari vulnerability," read the write up for one of the two just-patched vulnerabilities.

Firefox 3.0, although also patched, was at much less risk from the blended threat because of changes that limited access to local files from scripts running in the browser.

Mozilla also gave itself a plug in the advisory. "This attack only works if the user is using another Internet-connected application with Firefox not running. Using Firefox, or making sure it is at least running, prevents this attack," read Mozilla's description.

The second quashed bug, also judged critical, was in Firefox's CSSValue array data structure and could be used by hackers to force a crash, and from there, run malicious code.

That flaw also affects Thunderbird 2.0, Mozilla's e-mail client, which shares Firefox's rendering engine. Thunderbird, however, hasn't been updated since early May. In lieu of a patched Thunderbird -- something now scheduled to release July 23 -- Mozilla recommended that users disable JavaScript in the mailer.

Mozilla also reminded users that it will support Firefox 2.0 with security updates only until mid-December 2008, a warning it first gave last month after the debut of Firefox 3.0. Mozilla's standard policy is to support older software for only six months after the release of a major update.

Users can download Firefox 2.0.0.16 in versions for Windows, Mac OS X and Linux from the Mozilla site, call up the browser's built-in updater or wait for the automatic update notification, which typically appears within 24 to 48 hours.

Firefox 3.0.1, although not expected to appear until Wednesday afternoon Pacific time, will be posted to the Mozilla site when it does launch.