Windows has a file "auditing" feature that can track when files are touched, but this is not turned on by default because it creates absurdly voluminous logs. (Also, it might only be in the Pro version.) Without that data, I'm not aware of any way to track down the culprit. Perhaps someone on a Windows-specific forum would have more insight.

Web pages cannot clear a saved password. Web pages can clear their own cookies, but not the cookies of other sites.

Potential suspects would be cleanup utilities like CCleaner, and privacy tools that remove personal data (although, I would think only the cookies, not the passwords).

If you want to try to recover older versions of the database files that store your cookies you could try the Windows 7 "Previous Versions" feature.

Open your current Firefox settings (AKA Firefox profile) folder using

Help > Troubleshooting Information > "Show Folder" button

Leaving that window open, switch back to Firefox and Exit

Pause while Firefox finishes its cleanup, then copy the following files to a safe location in case you need to restore them:

• cookies.sqlite - cookies
• signons.sqlite - saved passwords
• key3.db - key file required to access signons.sqlite

Note: If Windows hides file extensions from you, turn off the hiding feature so you can work more accurately with file names. See this article for the steps: http://support.microsoft.com/kb/865219.

To check for and restore previous versions, right-click a file > Properties > Previous Versions.

Regarding the two password-related files, if they can't be restored from the same date, I'm not sure they will be usable.

Other possible causes are a corrupted cookies.sqlite file or maybe a problem with Session Restore (sessionstore.js stores cookies of open tabs).

• http://kb.mozillazine.org/Cookies