Sites that you never want to save logins for -- not the best English, but you know what I mean -- are on an "Exceptions" list on the Options page. That's where you should be able to remove the site so Firefox will ask you again the next time.

"3-bar" menu button (or Tools menu) > Options

In the left column, click Security. Then on the right side, over to the right of "Remember logins for sites" click the Exceptions button. Hopefully you can find and remove the site there.

Success?

Thanks. I've tried that, but nothing has ever shown in this section. Any other suggestions? Thanks for the quick response.

Hmm, maybe you haven't blocked Firefox from saving the password, so that's why it's not an Exception?

Is it possible the old/wrong login is still saved, and just has the wrong password? If so, you can edit that password.

Back on the Options page, Security panel, click the Saved Logins button, then click the button to Show Passwords and, if applicable, enter your Master Password to allow display. Double-click the current password to open a textbox for editing.

If that doesn't work, the other approach that comes to mind is very drastic: basically you remove all data about that site from Firefox (history, bookmarks, cookies) and as part of that, any permission-related data is also removed. It's the "Forget about this site" feature. Hopefully someone else will have a better/less destructive suggestion.

The other day I cleared my cache. I did as you suggested, and this time it actually let me edit the password. It wouldn't before; don't know why. Problem now solved. Thanks for the help.