I switched on my Win Vista with Firefox 3.6.12 this morning and it's forgotten some of my regularly used passwords for sites such as AdSense, Britannica and Statcounter yet WebmasterWorld and others seem unaffected.
I've not done anything, nothing new added or deleted, therefore anyone with any ideas as to what may have happened?
Did you go into Tools->Options->Security->Saved Passwords and click "Show Passwords" to see if they're physically missing or not?
I had an odd thing like that happen the other day and it was the auto-fill feature, not the passwords, messing up. Sometimes hitting reload did the trick, others I had to hit the down arrow in the user ID field to trigger the password.
Also, watch your status bar, passwords to seem to auto-fill only after the page is fully loaded and functional. If something is stopping the page from completing, it could impact the appearance of passwords.
If the password file was locked by another file (perhaps the previous version of the software?) and couldn't be unlocked, or a DB server task was down, or something of that nature. Rebooting resets all the file locks and resets all background tasks.
Rebooting is easily one of my first "go-to" fixes before I start digging into other potential causes.