The release of the third beta of Mozilla's Firefox 3.1 web browser has been delayed again after 18 bugs were detected in TraceMonkey - the browser's JavaScript engine.

Mozilla was unable to confirm whether the delay, the second one this month, would affect the releases of the final version of the browser.

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"I can't tell you that we're 100 percent confident that we will hit quarter one," said director of Firefox, Mike Beltzner.

"At this time, we don't have a good estimate for when we'll be done. Many of the bugs are proving to be tricky and complicated to fully resolve."

According to Beltzner, 15 of the TraceMonkey bugs have been described as 'Priority one blockers'. A Priority one blocker is a bug that, if unfixed, would prevent the release of Beta 3. He said once developers "get a good handle on the problem", the revised schedule should be posted within a few days.

"We'll check back with [the TraceMonkey team] in a couple of days, and see where they're at," he said.

"We really believe in the TraceMonkey engine. It's twice as fast [at rendering JavaScript] as Firefox 3.0, and more than nine times faster than Firefox 2.0. People who are using the nightlies and Beta 2 just can't go back to the slower browsers."

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Mozilla has made much of TraceMonkey, and the performance boost it gives Firefox, since it introduced the new JavaScript engine last summer.

Firefox 3.1 has been pushed back several times. Two weeks ago, Mozilla announced that Beta 3 would ship a week later than previously scheduled, on February 2. Last November, Mozilla inserted the third beta into its timetable to give more testing time to several features, including TraceMonkey.

"The TraceMonkey bugs seem quite containable," said Beltzner. "They're the sort of instability bugs that don't affect a lot of people a lot of the time - we're talking crashes that are affecting a small percentage of the web - but we don't want to crash on any."

Mozilla faces renewed pressure from Microsoft, which is working on the next version of its Internet Explorer browser. The company issued IE8 Release Candidate 1 (RC1) this week.


See also: Mozilla to track Firefox users