A change that Apple made in its latest update to Mac OS X forced Mozilla to issue a third release candidate for Firefox 3.0, according to notes from a status meeting the company held Tuesday.

As of today, Mozilla had seeded its download site with Release Candidate 3 (RC3).

The bug, which affected only the Mac version of the open-source browser, caused some systems to hang or crash at startup or shutdown. According to Bugzilla, Mozilla's bug-tracking and management system, the problem was traced to a change Apple made to "VerifiedDownloadPlugin" in Mac OS X 10.5.3.

Apple released 10.5.3 two weeks ago to fix more than 100 stability and security bugs.

Mozilla developers reported the problem to Apple. According to someone identified only as "philippe," who submitted the bug to Apple, the company's engineers e-mailed him to say: "After further investigation it has been determined that this is a known issue, which is currently being investigated by engineering."

However, Mozilla executives were unwilling to wait for a fix from Cupertino. "Anything we do in our application will be a workaround; any real fix has to come from Apple, on their timeline," Mike Shaver, Mozilla's chief evangelist, noted in a message yesterday on the Bugzilla thread discussing the patch. Rather than wait -- a potentially long wait, since others on Bugzilla confirmed that an unreleased 10.5.4 update to Mac OS X also contained the flaw -- Shaver created a workaround that prevented the at-fault plug-in from loading. Shaver's workaround was added to the Firefox code Tuesday.

No changes were made to the Windows and Linux versions of Firefox 3.0; instead, what had been set as last week's RC2 was simply renamed RC3.

Mozilla was even unsure about the purpose of the Apple plug-in. "I'm worried that the plug-in is part of Apple's download security scheme," said Ben Turner on Bugzilla a week ago. "Maybe it's this plug-in that is responsible for marking downloaded executables as coming from the Internet." But others, including Shaver, thought it safe to disregard VerifiedDownloadPlugin because Apple's own Safari Web browser doesn't bother to load it.

It's not clear how, or even whether, RC3 will affect the release schedule of Firefox 3.0; previously, company officials had set a mid-month deadline for shipping a final version. Mozilla did not reply to several e-mails and phone calls Wednesday.

Firefox 3.0 RC3 can be downloaded for Windows, Mac OS X and Linux in 43 languages from Mozilla's site.