Mozilla on Friday said it would update Firefox for the Mac to deal with a bug in Apple's impending Lion operating system.

Firefox 5.0.1 is "coming soon," Mozilla announced in a blog post, but did not set a release date. Users running Microsoft Windows or Linux will not see the update. According to Mozilla, Mac OS X 10.7 contains a bug that causes Firefox 5 to crash when displaying websites that use downloadable fonts.

"We alerted Apple to the problem before the release of 10.7 but they did not fix the problem before 10.7 went to final release," said Christopher Blizzard, Mozilla's web platform director. "We've changed the font APIs that we're using to newer versions which appear to fix the problem."

The bug in Lion will cause "severe crash problems" for Firefox 5 users if it's not fixed, added Blizzard.

Apple has slated Lion for release this month, and released a "gold master," the final that Blizzard referred to, on July 1. Apple has not announced the ship date for Lion, however. Some websites reported last week that Apple will deliver Lion this week, most likely on Thursday, July 14.

Although Mozilla moved earlier this year to a rapid release schedule that precludes most security or crash fix updates, which are instead rolled into the next version of the browser, it has repeatedly said in its developer mailing lists and meetings that it will issue an emergency fix if necessary. Lion's imminent release fits that bill, Blizzard said.

At the same time Mozilla releases Firefox 5.0.1 for the Mac, the company will also update the older Firefox 3.6 to disable downloadable web fonts when the browser runs on Lion.

Blizzard warned Firefox 3.6 users running Lion that they may notice another problem. "[They] may also see scrollbar rendering issues," he noted, then urged them to upgrade to Firefox 5. Mozilla is planning to retire Firefox 3.6 soon, perhaps as early as mid-August after it ships a final security update for the 18 month old browser.

Mozilla has already changed to the new font APIs in the Mac versions of both Firefox 6 and Firefox 7, the next two versions of the browser. Firefox 6, which reached beta status last week, is slated for an August 16 release, while Firefox 7, currently in what Mozilla calls its "Aurora" channel, is to ship September 27.