Mozilla has made major changes to how it develops Firefox, and plans to drop Firefox 3.7 from its schedule and instead roll out incremental changes throughout the year, a company executive said today.

Rather than add features to Firefox only in once- or twice-a-year upgrades, Mozilla will quietly insert some functionality via its regular security updates, which appear every four to six weeks, said Mike Beltzner, director of Firefox, in an interview Thursday.

That means Firefox 3.7, which was slated for a second quarter release, has been dropped from the development schedule, said Beltzner. The next version of the open-source browser after the almost-ready Firefox 3.6 will be an as-yet-unnamed update at the end of this year or in early 2011.

Last year, Mozilla said it would release a pair of fast-track updates, dubbed Firefox 3.6 and Firefox 3.7, that were to take about four or five months each to get out the door. Those so-called "minor updates" would then be followed by Firefox 4.0, a "major update" slated for the last quarter of 2010.

But based on the work on Firefox 3.6, Mozilla's changed its mind.

"I'm proud of how we challenged ourselves," said Beltzner, referring to the quick-strike schedule that Mozilla plotted out for Firefox 3.6, but didn't meet. Mozilla had slated Firefox 3.6 for a November 2009 release, but pushed back the ship date as it worked out bugs and added additional beta builds to the cycle. Firefox 3.6 reached the "release candidate" (RC) stage last week, and should wrap up before the end of this month.

"We learned an awful lot about what slows down our schedule, and that will help us plan future releases," Beltzner said.

In fact, the lessons from Firefox 3.6 led Mozilla to decide to can a release between now and the end of the year, the version previously tagged as Firefox 3.7. Instead, Mozilla will add behind-the-scenes capabilities to Firefox as part of the security updates it does on an irregular schedule every four or more weeks.

Mozilla's developers have been discussing that approach in various mailing lists for some time, and have codenamed the first such update as "Lorentz."

"The first target for [a minor update] will be separation of plug-in processes from the browser," said Beltzner. Mozilla is already working on a project, called "Electrolysis" where Firefox will run each tab as a separate process. The idea is to prevent a single site running in one tab from crashing the entire browser. Google's Chrome, for example, already has this capability.

Rather than wait to complete the entire Electrolysis project, Mozilla will instead separate the processes of specific plug-ins -- Adobe's Flash is the lead candidate -- so that if the plug-in crashes, Firefox itself isn't brought to its knees. According to Beltzner, Flash is responsible for more Firefox crashes than any other plug-in.

"This will be a huge advantage to users," said Beltzner, talking about the Lorentz concept. "We were thinking earlier that the first time we would be able to add [plug-in process separation] would be 3.7 in the middle of the year. But the change we need to make is very isolated, and has no effect on Web compatibility or add-on compatibility or on the user experience. So we thought, 'Why not deliver it as part of a minor update?'"

Up to this point, Mozilla's security updates, which are what Beltzner pegged as minor updates, have delivered patches and at times, stability fixes. They have not, however, added features, either visible or behind the scenes.

Not everything would be fair game for adding to a Firefox security update, Beltzner cautioned, but he said Mozilla was enthusiastic about the idea, which would let it add a "bunch of different technologies" to the browser without having to run through time-consuming beta cycles. Changes that require external testing, such as the planned interface revamp for Firefox on Windows Vista and Windows 7, would still be implemented only in major updates that would go through a standard beta test plan, Beltzner noted.

Firefox 3.6 RC, which is being run by almost one million users, looks "really good," Beltzner said when asked how it's progressing toward final. He acknowledged that there are still bugs developers are investigating, including some that may block a final release and necessitate another release candidate, and so declined to name a ship date.

In meeting notes published to the Mozilla site earlier this week, developers proposed two possible ship dates: Jan. 19 and Jan. 26.

Gregg Keizer covers Microsoft, security issues, Apple, Web browsers and general technology breaking news for Computerworld. Follow Gregg on Twitter at @gkeizer, send e-mail to gkeizer@ix.netcom.com or subscribe to Gregg's RSS feed .