The decision comes after several weeks of stalled progress on Firefox 4 Beta 7, which was originally scheduled to ship in mid-September but slipped as Mozilla ran into several stubborn bugs.
"Completing this work [on Firefox 4] is taking longer than initial estimates indicated as we track down regressions and sources of instability," acknowledged Mike Beltzner, director of Firefox, in a message posted to the mozilla.dev.planning mailing list.
According to a just-revised timetable, Firefox 4 will now shift to release candidate status sometime in early 2011. Release candidate, often simply dubbed 'RC', is the final stage of development before a software maker gives the green light for a final version.
A GA, or general availability, edition will ship shortly after one or more RCs are built and distributed for final testing, a Mozilla spokeswoman added in an email.
Firefox 4 Beta 7 is now set for release early next year, with two additional betas during November and another in December.
But that timeline could slip further. "We will not ship software before it is ready," Beltzner said.
Last May, Mozilla announced an aggressive development schedule for Firefox 4 that was to have produced a final edition by the end of November. At the time, Beltzner said that if Mozilla could not push Firefox 4 out the door by the end of November, the company would skip a December release and would release the browser in early 2011 instead, the path it outlined today.
"[December] is a bad time to release a product," Beltzner said then.
He also said that the increased number of betas, another hallmark of Firefox 4's development cycle that Mozilla touted last summer, will continue. "The frequent beta releases have been extremely helpful in identifying compatibility issues with existing Web content, so we plan on continuing to release beta milestones through the end of December," he said.
Firefox has a history of slipping its schedule. The current version, Firefox 3.6, for example, shipped in January 2010, about two months later than first planned.
In the last month, Mozilla has dropped some features from Firefox 4 that it originally intended to include in the upgrade. It's unclear whether the company will rethink those moves now that it's pushed the release into 2011.
Mozilla did not immediately reply to questions about whether it would revisit those decisions.
While Mozilla has delayed its next browser, rival Google has made good on promises to pick up the development pace of Chrome. Last July, Google said it would issue a new version of Chrome approximately every six weeks. Recently, Google shipped Chrome 7 seven weeks after Chrome 6's debut.
The current beta can be downloaded from Mozilla's site.