Use these steps to remove saved (form) data from a drop-down list:

1. click the (empty) input field on the web page to open the drop-down list
2. highlight an entry in the drop-down list with the mouse or cursor Down key
   do not click the mouse or press the Enter key
3. press the Delete key (on Mac: Shift+Delete) to delete the highlighted entry

You can also remove a stored password in the Password Manager.

• Tools > Options > Security: Passwords: "Saved Passwords" > "Show Passwords"

Details like websites remembering you (log you in automatically) are stored in a cookie. So you may have to remove those cookies.

That does not work. There seem to be 2 categories of password. One is automatically used when you send or receive mail. The other is the type that is requested in a dialog when Thunderbird goes to access the server. The dialog says that you can save this password but if you do so, the dialog just appears again at some stage when you go to use Thunderbird again. For the two years or so before I changed the server password, Thunderbird would never, ever, show a dialog asking for the password. I recollect I had to supply it when I Installed Thunderbird and Thunderbird just went on using it thereafter. Maybe the solution is to re-install Thunderbird - I just don't want to lose any of my settings, saved addresses etc. - especially the addresses that Thunderbird keeps automatically. Thanks for all your help so far!

Your question was tagged as Firefox, so if your question is about Thunderbird then I will move it to the Thunderbird program.