The move is the open-source browser maker's opening salvo against out-of-date, open-to-attack plug-ins from vendors like Apple, Adobe, and Sun.

One security expert applauded the news. "This is a great way of improving the security of Web browsers," said Wolfgang Kandek, chief technology officer at security firm Qualys, in a Saturday. "Flash is often used by attackers to exploit client machines and unfortunately notoriously difficult to update, requiring, on Windows, different update packages for Internet Explorer and all other browsers."

Firefox 3.5.3 and Firefox 3.0.14, security updates for the newest Firefox 3.5 and 2008's Firefox 3.0, respectively, are set to release Sept. 8. After installing either of those two updates, Firefox users will see a message if their computer has an out-of-date version of Flash Player. "You should update Adobe Flash right now," the message will read. "Firefox is up to date, but your current version of Flash can cause security and stability issues. Please install the free update as soon as possible."

The message will also include a link to the download site for the latest Flash Player plug-in.

"For now, our focus is on the Adobe Flash Player both because of its popularity and because some studies have shown that as many as 80% of users currently have an out-of-date version," said Johnathan Nightingale of the Firefox security team, in an entry to the .