Mozilla took the unusual step late yesterday of releasing an interim version of Firefox 3.5 in part because developers were unable to fix new bugs that have again delayed the official Release Candidate, according to a company executive.

Called Firefox 3.5 Preview, the build is being offered to the 800,000-some people currently running a beta of the browser upgrade. Users running the production Firefox 3.x won't see the Preview offer, however.

"We realized that we had 800,000 daily users of Firefox 3.5 Beta 4, and that those people could help us evaluate the majority of the changes that would be going into our release candidate, and consequently, our final release," Mike Beltzner, the director of Firefox, said in an e-mail reply Tuesday to questions. "The preview release updates those users to a product that is much closer to what we'll be shipping, and helps us get 1.6 million eyes on that product to ensure that we're not missing anything."

Mozilla developers found new bugs as they worked through the quality assurance and testing phase of the Release Candidate (RC), Beltzner added, which in turn pushed back the RC's expected post date from to early next week. It was expected to be out by the end of this week.

"We think that we'll be closed on code imminently -- the issues we're discovering are smaller and smaller and easier and easier to fix -- but before releasing the Release Candidate we must put it through a full quality assurance pass, which is a multi-day process," said Beltzner. "This was another reason for the preview release: it essentially grows our quality assurance team by 800,000 people, allowing us to leverage the power of our community."

Last week, Beltzner had pegged June 10-12 as the likely ship date for Firefox 3.5 RC. That date was also a delay from an earlier estimate.

It's becoming less likely that Mozilla will meet its end-of-this-month deadline for Firefox 3.5's final code. Last year, Mozilla delivered the final release candidate of Firefox 3.0 on June 4, then launched the browser two weeks later, on June 17. Assuming it sticks to that same timeline this year, Mozilla could ship Firefox 3.5 by June 28-30 if it limits itself to a single RC and drops that on users between next Monday and Wednesday.

Firefox currently accounts for 22.5% of the browser market, according to Web metrics company Net Applications' most-recent data. But it faces renewed competition on almost every front, including Microsoft and its Internet Explorer 8, Google's Chrome browser, Opera Software's Opera and Apple's Safari 4, which launched in final form for both Mac OS X and Windows yesterday.

Users running Firefox 3.5 Beta 4 should receive an offer to update to the Firefox 3.5 Preview no later than the end of today. They can also choose "Check for Updates" under the "Help" menu to do a manual update.