Deleting autocomplete entries
From MozillaZine Knowledge Base
To delete individual entries from the Firefox Location Bar, Search Bar, and other form field drop-down lists:
The Up and Down arrow keys, and "Page Up" and "Page Down" all work to activate autocomplete or to move in an Up or Down direction within Autocomplete both selecting (highlighting) and copying the line into the Location Bar.
Note: In SeaMonkey, you can delete individual entries from the Search Bar and from other form field drop-down lists using the above steps, but not from the Location Bar (bug 87098).
You can delete your entire autocomplete lists from the Firefox "History (or Tools) -> Clear Recent History" menu ("Tools -> Clear Private Data" in SeaMonkey)
You can delete login-names and passwords in the Password Manager list of saved (remembered) passwords. Select the items that you want to delete and click the "Remove" button (or press the Delete key). See the Password Manager article for details.