The Mozilla Foundation has delayed the release of the next major version of Firefox until September, a move that followed recent troubles in the release of bug fixes for the popular open-source browser.

Firefox 1.1, code-named Deer Park, had been scheduled for release this month, but changes in the product's roadmap was announced in a posting by developer Ben Goodger on the Mozilla website.

The updated timetable has Firefox 1.5 shipping in September, with versions 2.0, code-named The Ocho, and 3.0 tentatively scheduled to ship in the first quarter and third quarter of next year, respectively. Version 1.1 has been scrapped in favor of 1.5, Goodger said.

A version 1.4 general preview release of 1.5 is scheduled to ship in August.

"This is, as always, subject to change," Goodger said in his Wednesday posting.

This is at least the third delay in the next major release of the browser. Version 1.1 was scheduled to ship in March, before it was postponed to June and then July.

The delays have been caused by an increasing number of features Mozilla has decided to add into the next release, Asa Dotzler, community coordinator for the organization, said. Among the biggest changes is in the update system that will be part of version 1.5.

The new system would make it possible to patch the browser with small-sized upgrade files, rather than force users to reinstall a full version of the browser. In addition, the next version is expected to have support for scalable vector graphics and a default feature called Fastback that caches previously visited pages in memory for faster display when clicking the back and forward navigation buttons. Other new features include the ability to group tabs that let the user move between multiple pages in the same browser, and one click removal of cookies, history and temporary Internet files.

The long feature list has added to the development time, making the delay unavoidable, said Dotzler, who also pointed out that the original launch dates were never final.

"We always qualify that we're not going to ship (the browser) until it's ready," Dotzler said.

Of course, when something is actually "ready" is sometimes relative. Mozilla this week updated the current versions of Firefox and the Thunderbird email client for the second time in eight days to fix flaws. Version 1.0.6 of Firefox patched bugs in version 1.0.5, which had fixed security vulnerabilities.

Last week, Mozilla posted the second alpha edition of Firefox 1.1, which included many of the features described above.