The company said the new technology, dubbed 'Content Security Policy' (CSP), is a Mozilla-initiated specification targeted at website and application developers, who will be able to define which content on the site or in the online application is legitimate.
That would block any script or malicious code that's been added by hackers who manage to compromise the site or app. Such attacks are generally tagged with the label of cross-site scripting (XSS).
Preview editions of Firefox are available for developers to try out, said Mozilla in an announcement last week .
"This isn't a single trick that's meant to counter a single kind of attack," said Johnathan Nightingale, the manager of the Firefox front-end development team.
"This helps sites solve cross-site scripting, but it's more than that. They now have a way to shut everything dynamic off, so that no matter what content gets added to a site, if it's on the page and they've sent us policy instructions in its header, we shut it down."
Firefox is passing the baton to site and application developers, who will be able to separate the legitimate from the illicit content. With CSP in place, Firefox will allow the former but will automatically block the latter.
"The main difference is that the website itself is determining the policy. NoScript is a great tool, but a large number of web users are not sophisticated enough to manage the kind of decisions it requires."
Nightingale and Sterne have pinned high hopes on CSP, which grew out of an idea first put forward by security researcher Robert 'rsnake' Hansen in 2005.
Last year, Hansen, the CEO of SecTheory, and Jeremiah Grossman, chief technology officer at WhiteHat Security, made headlines when they revealed details about how browsers were vulnerable to so-called 'clickjacking' attacks.
"Absolutely, this will drive a stake through the heart of cross-site scripting attacks," argued Sterne.
"An attacker injects some script that harms the users of that site, that encompasses content injection. Out of the box, CSP [lets sites send] signals to the browser that says, 'We're gonna turn off everything by default.' Cross-site scripting will be neutered at that point."
But Nightingale and Sterne realise that, even with nearly a quarter of the world's internet users running Firefox, Mozilla faces a tough road if it's the only browser maker pushing CSP.
"Both the Internet Explorer and Chrome teams have contributed to the design discussions of the specification," said Sterne. "They have some tentative interest in implementing it at some point in the future."
Earlier this year, Eric Lawrence, a program manager on Microsoft 's Internet Explorer (IE) team, called CSP "a good idea" and "a promising approach" in a pair of entries on the official IE blog, but did not commit Microsoft to supporting the technology.
"It's great to see that others are taking this threat seriously, as well," said Sterne.
Google, the maker of Chrome, was not available over the weekend, but the company has previously said it generally doesn't comment on future product development.
Mozilla must also convince site and application developers that it's worth their while to use CSP. Nightingale and Sterne declined to name the sites that have expressed interest in using the technology.
"The first step is for us to use it," said Nightingale, adding that Mozilla would turn one of its online properties into a guinea pig to show others that CSP is possible, and to iron out problems.
The pair was also vague about when CSP would debut in a production version of Firefox. The one thing they did say was that it wouldn't show up in the minor upgrade, Firefox 3.6 , that's to ship in November. The first, and likely only, beta of Firefox 3.6 is slated to ship October 13.
"Whatever comes after 3.6, that's the earliest," said Sterne.
Mozilla isn't the only browser maker trying to protect users from cross-site scripting attacks. Microsoft, for example, added a cross-site scripting filter to IE8 that the company said would block most attacks.
Preview builds of Firefox with CSP enabled can be downloaded for Windows, Windows Mobile, Mac and Linux from Mozilla's server. Sterne has also posted a demonstration page that graphically shows how various scripts are blocked by the technology.