It's looking increasingly unlikely that Mozilla will ship Firefox 4 this month, after Christian Legnitto, who oversees Firefox releases, revealed that Beta 12 will probably not ship for several days.

"The bugs blocking beta 12 are expected to be fixed in the next day or so," Legnitto said in a message to a Firefox development mailing list . "At that point, we will freeze nightlies and then create the beta build when we are confident of quality."

Last week, five bugs remained on what Mozilla calls its 'hard' blocker list - those bugs that must be fixed before it deems Beta 12 good to go.

One of the five bugs is rated 'critical', while another is ranked 'major' and three are labelled 'normal'. Two of the remaining flaws are in Firefox's hardware acceleration code, which shifts some of the browser page rendering and composition chores from the CPU to the graphics processor.

Although Mozilla had once planned to ship Firefox 4 in November 2010, delays last autumn forced it to announce in October that it would instead wrap up development early this year .

Last month, Damon Sicore, Mozilla's director of platform engineering, said that the open-source company was shooting for a February release of the Firefox 4 final code. At that time, Sicore said 160 hard blockers remained, and that developers would "ideally finish the hard blockers by the beginning of February".

Legnitto also left open the door to yet another beta, which would delay a 'release candidate' build even further.

"Beta 12 is currently planned to be the last beta (by definition due to not having any beta hard blockers after it is created), but we still might need a beta 13 if issues found in beta 12 need beta coverage before a release candidate," he said on Thursday.

Web browser turf wars

Mozilla faces pressure from other top browsers. Microsoft, which shipped Internet Explorer 9 (IE9) Release Candidate (RC) last week, has sent out invitations to the press for a March 14 event at SXSW (South by Southwest Conferences and Festivals).

Microsoft, which mailed similar invitations in both September 2010 and January 2010 for events that later included roll-outs of IE9 Beta and IE9 RC, respectively, may use the SXSW conference to launch IE9's final build.

Previously, Microsoft promised to ship IE9 in the first quarter of 2011.

Meanwhile, Google releases a new version of Chrome about every six to eight weeks. The latest version, Chrome 9, shipped on February 3.

Mozilla has tacitly acknowledged that it needs to step up the pace. In a proposed roadmap for 2011, Mike Beltzner, director of Firefox, laid out an ambitious schedule that may result in as many as three more upgrades - Firefox 5, 6 and 7 - during the year.

Firefox currently accounts for 22.8 percent of all browsers in use, according to web metrics company Net Applications.

Firefox 4 Beta 11, which at time of writing was the latest preview, can be downloaded for Windows, Mac and Linux from Mozilla's site.

See also: Firefox 4 startup time boosted by patch