Mozilla will add a new lockdown feature to Firefox 3.6 that will prevent developers from sneaking add-ons into the program, the company said.

The new feature, which Mozilla dubbed "component directory lockdown," will bar access to Firefox's "components" directory, where most of the browser's own code is stored. The company has billed the move as a way to boost the stability of its browser.

"We're doing this for stability and user control [reasons]," said Johnathan Nightingale, manager of the Firefox front-end development team, in an e-mail today. "Dropping raw components in this way was never an officially supported way of doing things, which means it lacks things like a way to specify compatibility. When a new version of Firefox comes out that these components aren't compatible with, the result can be a real pain for our shared users.

"Now that those components will be packaged like regular add-ons, they will specify the versions they are compatible with, and Firefox can disable any that it knows are likely to cause problems," Nightingale added.

His mention of "regular add-ons" referred to the new policy that will be enforced by Firefox 3.6, a minor upgrade to last summer's 3.5 that is to ship before the end of the year. Because third-party developers will no longer be able to drop their code into the components directory, they must instead recreate their add-ons as XPI-based files, the standard Firefox extension format. Mozilla has posted information on its developer site to aid programmers who need to migrate add-ons to the XPI format.

Most, but not all, Firefox add-ons are available through Mozilla's Add-On site, which boasts that more than 1.6 billion add-ons have been downloaded by users.

Nightingale said that rogue add-ons created performance and stability problems for Firefox users. "[They] can lead to all kinds of unfortunate behavior: lost functionality, performance woes and outright crashing, often immediately on startup," he wrote in a post to the Mozilla developer's blog Monday.