Mozilla developers have released an updated version of their Firefox browser that fixes a number of security issues, four of them rated critical.

The update was released land Firefox users should receive the 1.5.0.7 patches via the browser's automatic update system over the next few days, according to Mozilla.

Research firm Secunia rates the flaws as "highly critical," saying that they can be exploited to "conduct man-in-the-middle, spoofing, and cross-site scripting attacks, and potentially compromise a user's system," according to an alert from the company.

Web surfers who want to download the Firefox update directly can find it online.

In addition to the four critical fixes, the software also corrects three less-critical issues and offers some stability enhancements, according to the Mozilla Web site. Details on the security vulnerabilities are now available online.

Browser hackers have traditionally focused on Microsoft Corp.'s Internet Explorer, but as Firefox's market share has grown it has become a more attractive target. Firefox is now used by about 13% of Web surfers, according to research firm OneStat.com.

It has been a busy week for IT administrators looking to stay on top of security. Important security patches were also released by Microsoft Corp. and Apple Computer Inc.

On Tuesday, Microsoft released its monthly set of security patches, fixing a critical bug in its Publisher product. The company also re-released two of its August patches to fix bugs in the updates.

That was followed Wednesday by Apple's fix for its QuickTime multimedia software. This update, rated as highly critical by Secunia, fixed six bugs in the product. More details on the QuickTime update can be found on Apple's Web site.