Jetpack, the revamped foundation for add-ons that customize Firefox, could arrive in its first beta form Monday.

Myk Melez, a leader of the project, issued a release candidate yesterday for the software and hopes to release the beta Monday, he said in a mailing list announcement.

"To have the most successful possible release, it's imperative that we test the hell out of this release candidate to shake out any last remaining release blockers," he said, referring to bugs that are bad enough to hold up the software.

Jetpack, taking a page from the Google Chrome extensions playbook, is a mechanism to let developers write add-ons using Web programming technologies such as HTML, JavaScript, and CSS. Apple followed suit with its own similar extensions mechanism in Safari 5, and Opera is doing the same with Opera 11, currently in beta testing. The approaches are similar enough that Opera Chief Technology Officer Hakon Wium Lie believes that the browser extension framework eventually could be standardized.

Extending browsers lets people significantly expand what the software can do without burdening the vast majority of users with undesired features. What Mozilla calls add-ons its rivals generally call extensions. They're separate from plug-ins such as Adobe Systems' Flash Player or Unity 3D's gaming engine, which integrate through a different interface.

Firefox has had add-ons for years, but using a more complicated technology called XUL. It's sophisticated to permit very elaborate modifications to the browser, but Mozilla believes Jetpack will make add-on programming easier and more accessible. It also hopes the add-ons will be less prone to a major problem with today's add-ons, incompatibilities that break them when new versions of the browser arrive.

The Jetpack 1.0 beta won't have all possible interfaces Mozilla hopes to open up to developers. However, Mozilla expects to keep those interfaces it does have stable in future versions, Melez said.