Previous schedules published by Mozilla had limited Firefox 3.1 to only two betas before moving to a release candidate.
In a long post to the mozilla.dev.planning forum, Mike Beltzner, the director of Firefox, said that Beta 3 was necessary to get a feel for the severity of the remaining bugs and an idea of how long it will take developers to eradicate them. In addition, another beta will give more exposure to features landing in the browser only as of Beta 2, which has not yet been released.
Beta 3 is not a done deal, Beltzner noted in a follow-up email, but he is confident that developers would approve the plan.
"We're never comfortable declaring new milestones by fiat, but I expect that there won't be any opposition to the plans for a third beta at today's meeting," he said. "I'd say that it's very likely at this point."
So far, Mozilla has shipped only Beta 1, which was released six weeks ago, although Beta 2 should be available in early December, perhaps as soon as the end of next week. A schedule for the third beta has not been set, but Beltzner said Mozilla would likely declare a "code freeze" - a milestone after which changes are either forbidden outright or tightly restricted - in early January.
Beltzner stressed that the extra beta wouldn't delay the final version of Firefox 3.1.
"We believe we can do this without major impact to our shipping schedule," he said.
"It's more a matter of inserting another public consultation milestone than it is about slipping, per se."
Mozilla is traditionally leery of committing to final ship dates - like other developers it typically says it launches products when they're ready, not on a timetable - but previously it had said it was shooting for a late 2008 or early 2009 window.
Yesterday, however, Beltzner said that Firefox 3.1 is "still looking at late in Q1 2009 for final delivery".
In a status meeting last week, Mozilla also decided to retract a revamped Ctrl-Tab tab-switching feature it had originally slated for Firefox 3.1.
The enhancement, which was based on an already-available Firefox add-on, showed users thumbnails when they cycled through open tabs, and switched between current and last-viewed tabs rather than simply moving to the next tab to the right.
Like many of the features that made it into Firefox 3.1, it was initially set for Firefox 3.0, but had slipped out of that earlier update.
"This is something that our development community is getting used to," Beltzner said Tuesday in another email.
"As part of our new effort to try and increase the pace of our releases, we're coming to terms with the fact that new user-facing features sometimes need a lot of trial-and-error to get feeling right."
The newest attempt at redesigning tab switching, said Beltzner, didn't "feel quite right" to either Mozilla's user interface team or outside testers; the feature was pulled as a result.
"If we can get it right, we can see if we still have time for its inclusion," he added.
"If not, it can wait until the next release."
Mozilla was the second browser developer in the last week to claim it is still on schedule. Last Thursday, Microsoft laid out the next step for Internet Explorer 8 (IE8); Monday, a Microsoft director of product management denied that IE8's schedule had slipped in an interview with the Seattle Post-Intelligencer.
"It's not a slipped schedule," Matthew Lapsen, a director of Windows product management, told the newspaper. "We release based on product quality, not dates."
Some Microsoft officials, however, had alluded to a late-2008 ship date for IE8 several months ago.