According to Mozilla, the problem stemmed from the fact that the Password Manager has issues with the login records featuring international characters. To be more precise, the browser was able to save correctly non-ASCII data only in UTF-8.
The release is rather odd for Firefox fans, seeing that Mozilla updates usually come in both Firefox 2 and Firefox 3 flavor. However, this time the bug seems only to affect the latest version of the browser.
The Firefox 3.0.3 update can be downloaded from here.