Plugins are small bits of code that enable other applications to run within a web browser, such as Adobe's Flash multimedia program.
However, outdated plugins are a danger for internet users, as they can contain software vulnerabilities that can be used to gain remote control over someone's PC.
The problem has been recognised for some time, and some vendors have incorporated automated software update alerts to let users know when either they need to download a new plugin.
But others have no such mechanism.
Each web browser requires its own plugin, and if a user doesn't know a new version is available, they're unlikely to update it, which puts them at risk.
Mozilla said studies have shown that up to 80 percent of users have an outdated plugin.
The latest version of Mozilla's Firefox browser, version 3.6, automatically checks for outdated plugins, but other browsers do not.
For other browsers, Mozilla has built a web service that will scan to see if several types of plugins are current.
"We believe that plugin safety is an issue for the web as a whole, so while our initial efforts focused on building a page that would work for Firefox users, the team has since expanded plugin check coverage to work with Safari 4, Chrome 4 and Opera 10.5," said Johnathan Nightingale, director of Firefox development, in a blog.
"We have added support for Internet Explorer 7 and 8 for the most popular plugins as well, but since IE requires specific code to be written for each plugin, it will take us a little longer to get to full coverage," he said.
Mozilla launched the plugin project for its own Firefox browser last fall, focusing on Adobe's applications.
After installing security updates, Firefox versions 3.5.3 and Firefox 3.0.14 would display a warning if the company's Flash player was out of date.
For Firefox, the new page will check for plugins such as Apple's QuickTime and Microsoft's Silverlight multimedia players, Adobe's Acrobat reader and Flash plugins plus others.
For IE8, the page scans for Silverlight, Windows Media Player, Flash and QuickTime so far.
Mozilla is also appealing to software vendors to contribute to a plugin directory that will track when new versions are available.
"If you're a plugin vendor, we need your help," Nightingale said.
"The directory is currently in alpha stages, and we need vendors to let us know as new versions come out and old versions become dangerous."