Hi Jason-watson, there are two ways a username or password might be filled (not counting add-ons, there are many add-ons...):

Autocomplete based on forms history: the arrow down and Delete method clears this kind of entry but on Mac, you might need to use Shift+Delete. See the "Deleting individual form entries" section of Control whether Firefox automatically fills in forms. (You can adjust the article for your OS using the "Editing Tools" in the left column.)

Password Manager: if you are using this feature, you can delete saved logins on the Preferences page:

"3-bar" menu button (or Firefox menu) > Preferences

In the left column, click Security. Then on the right side, find the "Saved Logins" button. Click that to open a dialog which allows you to remove individual saved logins.

Sorry, why did I think you were using a Mac? Once again, for Windows:

Hi Jason-watson, there are two ways a username or password might be filled (not counting add-ons, there are many add-ons...):

Autocomplete based on forms history: the arrow down and Delete method clears this kind of entry. See the "Deleting individual form entries" section of Control whether Firefox automatically fills in forms. (You can adjust the article for your OS using the "Editing Tools" in the left column.)

Password Manager: if you are using this feature, you can delete saved logins on the Options page:

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

In the left column, click Security. Then on the right side, find the "Saved Logins" button. Click that to open a dialog which allows you to remove individual saved logins.