I see know reason why an automatic update would delete yahoo passwords.

Manual updates and reinstalls may delete settings, often due to the users picking the wrong option.

What exactly is it you lost ?

• Does it include site logins and user passwords that Firefox stores ?
• Were you using a Firefox master password ?

Do you mean names and passwords in the Password Manager or do you mean that you are no longer logged on to (remembered by) websites after closing and restarting Firefox?

There is a difference between remembering the name and password in the Password Manager and a "remember me" check box on a web page to log you in automatically.
The latter usually involves the creation of a special "remember me" cookie that is stored on your computer and that is send to the server.
If that "remember me" cookie is removed or gets corrupted or is not send to the server then the server won't remember you anymore and you need to sign in once again.

• http://kb.mozillazine.org/Cookies