Magento introduced a new security feature in version 1.8 which disables the possibility to save and autocomplete the admin credentials. Some password manager like Lastpass can still save and autocomplete the credentials, but some others cannot. For instance, the Firefox password manager does not store or autocomplete the admin credentials in Magento >= 1.8. I wrote a neat little extension to solve this issue.
The extension is, as always on GitHub. But this one is also on Magento Connect (woho!).