Mozilla has delayed the third beta of Firefox 3 .1 for the second time this month according to a company executive. He cited troublesome bugs in the browser's new JavaScript engine as the reason.

It's not yet clear if the latest delay will affect the delivery of Firefox 3.1's final, which Mozilla has said several times would appear this quarter. "I can't tell you that we're 100 percent confident that we will hit Q1," said Mike Beltzner , director of Firefox.

After a Firefox 3.1 status meeting Wednesday, Mozilla noted that there were 18 bugs that still need fixing before it can move ahead with Beta 3. "At this time, we don't have a good estimate for when we'll be done," meeting notes read. "Many of the bugs are proving to be tricky and complicated to fully resolve."

Beltzner expanded on that theme. "The TraceMonkey team has 15 things that are priority 1 blockers," he said, referring to the JavaScript engine that Mozilla introduced last year in Firefox 3.1. A Priority 1 blocker is a bug that, if unfixed, would prevent the release of Beta 3.

Saying that TraceMonkey developers needed to "get a good handle on the problem," Beltzner said a revised schedule might 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.

There has been no talk of yanking TraceMonkey from Firefox 3.1, Beltzner said. "We really believe in the TraceMonkey engine," he confirmed. "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," he said.

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 on 2 February a week later than previously scheduled . 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 [sites] -- 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. On Monday, Microsoft issued IE8 Release Candidate 1 (RC1).