"Tools > Options" is for Windows, and "Edit > Preferences" is for Linux. The equivalent on the Mac is "Firefox > Preferences"

http://kb.mozillazine.org/Menu_differences_in_Windows,_Linux,_and_Mac

For how to delete passwords see the remembering passwords article. The "remember me" options are normally stored in cookies, try deleting Yahoo cookies, for details see deleting cookies.

Thank you! I had no idea how to sort it and your answer worked. You've saved me a lot of time.

Performing a disc cleaning will remove all stored user names and passwords you have in Sxipper along with all the garbage.