The latest releases of the Firefox browser and Thunderbird email client break extensions built by third-party developers, and new versions of the flawed software are expected to be released this week, the Mozilla Foundation said.

The creator of the open-source software said in its MozillaZine blog that the upcoming 1.0.6 release would replace 1.0.5, which shipped about a week ago. The latest update was needed to fix changes in the application-programming interfaces of 1.0.5 that "unintentionally broke some extensions."

For example, Enigmail PGP, security software that enables email encryption and other features, does not work in the 1.0.5 release of Thunderbird, Mozilla said.

Chris Hofmann, director of engineering for Mozilla, said problems with third-party applications are common across the industry during software upgrades.

"Backward compatibility with any kind of application development is kind of the holy grail of software problems that haven''t been solved by anyone yet," Hofmann said. "(But) you try to respond quickly when it does happen."

The problems with 1.0.5, however, do not affect browser users, but only a "limited" number of web content providers and application developers, Hofmann said.

"But all these groups are important to us, which is why we are moving quickly to make these changes," Hofmann said, noting that the next release, 1.0.6, would be available in a couple of days.

The latest changes also appear to have delayed the development of foreign-language versions of Firefox and Thunderbird. Version 1.0.5, which fixed a number of security problems, is available only in English.

Development teams building foreign-language versions have complained of a lack of "clear information" from Mozilla on the latest problems. The foundation has asked developers to skip localized versions of 1.0.5 and to build them instead for the upcoming version.

The changes have frustrated some developers, who have complained that Firefox users are upset with the delay in shipping foreign-language versions.

"We are getting lots of e-mails from Firefox users in Poland asking us about why isn''t Firefox 1.0.5 available in Polish. ... The journalists are also asking us the same questions... a few days more and it''s gonna be a big PR (public relations) disaster for Firefox outside the USA," Marek Stepien said, according to the Mozilla blog.

In the past, developers have been able to ship versions of Firefox for as many as 35 languages within a week of a browser release, Hofmann said. That high expectation has added to the anger over the current problems.

"There is some frustration that this one is going a little slower, and we''re frustrated with that as well," Hofmann said.

Version 1.0.5 fixed several potential security problems, based on vulnerability reports from private security researchers participating in the Mozilla Security Bug Bounty Program and from security companies such as the Danish firm Secunia.

Mozilla also released this month the second alpha edition of Firefox 1.1, which includes an integrated update system that will make it possible to patch the browser with small-sized upgrade files. Users today are forced to download and reinstall full versions of the browser in order to update the software.

Alpha 2 also includes a feature that caches previously visited pages in memory for faster display when clicking the back and forward navigation buttons.