The bug prevents net surfers from using saved usernames and passwords if they contain certain characters, and was discovered soon after the Firefox 3.0.2 software shipped earlier this week.
The bug also leaves users unable to access their existing saved passwords or create new ones, said Mike Beltzner, director of Firefox development.
It's not that widespread. We don't expect it to be affecting many people," he said.
The Firefox quality assurance team is now testing a bug-fix and expects to release the 3.0.3 update early next week, he said. No other bugs are expected to be patched in this update.
The 3.0.2 update includes nearly a dozen security fixes for the open-source browser, six of which are rated as critical.