Lately i thought i had picked up a virus and i would log into yahoo on firefox. But if i closed the window and came back to it (even having clicked save password/user name) Firefox would ask for them again.

Also this is happening with my bank log in.

I have checked on Internet exploerer and it is not doing this. It is keeping the log ins.

Did you set firefox up to clear all personal data when you exit? If so it will clear all the passwords and log-in names when you exit, go into the settings and make sure you don't have it set to clear all personal data, also a lot of these are saved as cookies so if you have it set to clear cookies on exit this may be the problem. You can always uninstall Firefox and download and install the latest version in case it is corrupted.