As Mozilla began the final push on Firefox 3.0 Beta 3 this week, it also announced that at least one more beta will be necessary before it starts building release candidates.

On Tuesday, Mozilla "froze" the code for Firefox 3.0 Beta 3. Testing of the beta is scheduled to start on Monday.

At the same time, in a message to the mozilla.dev.planning newsgroup, Mike Beltzner, Mozilla's interface designer, said that a fourth beta would be required.

"[We] will be adding another milestone before moving on to Release Candidate builds," Beltzner said. "Our goal is to do a quick turnaround on Firefox 3 Beta 4, but we cannot provide a good estimate until we know the size and scope of blockers remaining after the Beta 3 code freeze."

As of Beta 3, all efforts will turn to addressing bugs that could stymie the preview's release, Beltzner added. "We will be driving the list of blocking+, P1, target=beta 3 bugs to zero," he said. As of Thursday there were three bugs that met his criteria; in Mozilla's nomenclature, such bugs are dubbed "blockers," meaning they are crucial enough that they require solutions before the beta is allowed to ship.

According to notes from a Tuesday meeting, Beta 3's tentative schedule points toward a release on or about 13 February.

Those same notes also indicated the status of efforts to make the browser's add-ons compatible with the upcoming preview. Approximately 41 per cent of all extensions, by usage, have been modified so that they will work with Beta 3, Mozilla reported. Several widely used add-ons, however, do not yet support Firefox Beta 3, including IE Tab, Forecastfox, Fasterfox and Firebug.

Mozilla released Firefox 3 Beta 2 on 18 December, beating a self-imposed deadline by several days. The company regularly declines to set a release date for the final version of the browser.