From MozillaZine Knowledge Base

The Location Bar, Search Bar and some form fields on web pages have autocomplete dropdowns that appear with a list of previously-entered data. To delete individual entries:

1. Click on the text field
2. Press down to activate autocomplete
3. Highlight the entry you want deleted by using the arrow keys or hovering over it with your mouse
4. Delete (In Firefox 1.5, Shift + Delete).

You can delete your entire autocomplete lists by using Tools -> Clear Private Data.

• Checking "Browsing History" will clear the Location Bar autocomplete (as well as your history)
• Checking "Saved Form and Search History" will clear web pages' autocomplete and the Search Bar's autocomplete

Deleting passwords

You can delete login-names and passwords in the Password Manager "Tools -> Options -> Security / Passwords -> Show Passwords". Highlight (select) the items that you want to delete and click the "Remove" button (or press the Delete key). See the Password Manager article for additional information.

See also

• Disabling autocomplete - Firefox