Please explain the problem in detail. What happens? What is/are the exact error message(s) ?

Is the problem that the Password Manager is providing the wrong information during login to that site? Try entering the information manually Firefox should then ask if you want to save the new login.

Thank you for your reply. I go to the optus webmail page through Firefox. Log in with my new address & current password, an error message says "Login failed- user name or password incorrect". I use my old email address & the same password & it opens. Trouble is, I only get the emails that are still going to my old address, not the new one. As I said, Optus believe it's a browser issue & said I may have to uninstall & reinstall Firefox. That seems a bit drastic, shouldn't there be some way of changing it? Maybe I'm missing something obvious?

Try using Windows Internet Explorer or Edge browsers. Is the problem still there?

Delete browsing, search and download history on Firefox {web link}

Remove a single website from your history {web link}

Open the History Manager <Control> (Mac=<Command>) <Shift> H. In the search bar, enter the name of the site. Right-click on one of the listings and select Forget About This Site. This should remove all information, including any site settings And Passwords.

Thank you again, I'll give it a try.

You can add a new login to Lockwise with the new email address and the correct password.

• https://support.mozilla.org/en-US/kb/password-manager-remember-delete-edit-logins

Note that you may have to clear the cookies for this website and possibly clear the cache as well.

• Settings -> Privacy & Security
  Cookies and Site Data: "Manage Data"

"Clear the Cache":

• Settings -> Privacy & Security
  Cookies and Site Data -> Clear Data -> [X] Cached Web Content -> Clear

Unfortunately that didn't solve the problem. I'm thinking I might have to go back to Optus because I received an email to my new address with my old address displayed in the "sign in" area. Very strange, but I'm sure there's a simple solution, somewhere! Thank you for your assistance. I'll come back to you when I eventually get it solved.