Firefox 3.0's first beta has been delayed at least six weeks, Mozilla engineers said Sunday, and it now won't appear until the middle of September.

In a posting to the mozilla.dev.planning group Sunday, Mike Schroepfer, Mozilla's vice president of engineering, suggested a revamped milestone schedule for Firefox 3.0, the next major update to the popular open-source browser. Among the changes: a postponement of the first beta, slated as recently as last week to debut July 31. It's not expected until Sept. 18 at the earliest.

"Based on this criteria, it does not appear that M7 will be ready to be called a beta," Schroepfer wrote. In his post -- which he described as a written version of verbal discussions held last week -- Schroepfer used "M" designations to label milestones, which could be dubbed alphas or betas, depending on their stability. M6, for example, was the tag placed on Firefox 3.0 Alpha 6, the preview released July 4; at the time, that version was to be the last alpha before beta testing began.

"Talos is showing a ~18 percent increase in [memory] footprint and informal dogfooding confirms things are currently worse on the trunk," Schroepfer continued, referring to Mozilla's performance testing project, dubbed Talos. The term "dogfooding" refers to developers using their own under-construction software.

In addition, several features intended for the Firefox 3.0 final have yet to make it into the code, including an anti-malware tool and the polished application programming interfaces needed to support offline Web software.

The revised schedule published Monday spells out dates for Alpha 7, as well as M8 and M9. The last two may or may not be betas. "We'll switch from Alphas to Betas as soon as we believe Firefox is stable and usable enough for daily browsing for a large number of people," said Schroepfer.

Dates beyond the Oct. 16 code freeze of M9 are up in the air. "When will the last Beta ship? As soon as it is ready," Schroepfer wrote.

Neither Schroepfer's comments nor the revamped calendar mentioned anything about Firefox 3.0's launch date, which has in the past been set as sometime this year. Mozilla was not immediately available for comment on whether the changes will push the final release into 2008.