Bug #451155 found in the recent Firefox 3.0.2 release has been found to prevent access to saved passwords that include international characters either in the web address the password is saved for, the login, or the password itself. A firedrill update (Firefox 3.0.3) will be released sometime next week.
Mike Beltzner, Firefox lead, explains in a post to the mozilla.dev.planning group:
The symptom is that users who have password data stores with non-ASCII data saved as something other than UTF-8 (more common for people who have saved passwords on IDN domains or non en-US domains) will not be able to access their saved passwords or create any new saved passwords. There is no permanent dataloss, the saved data is just inaccessible. While this doesn’t affect all Firefox users, it is a significant regression and has triggered a fast-release Firefox 3.0.3 which will contain a single fix for this issue.
Source: Mozilla Links