Author: Edmund Laugasson
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)
If you're filling out a form and you see an Autofill suggestion, you can
delete it on the page.
In the dropdown, use your keyboard's arrow keys to select the Autofill suggestion.
Windows: Press Shift + Delete.
Mac: Press Shift + fn + Delete.
In the top right, click the icon you see: Menu Menu or More More.
Click Settings and then Show advanced settings.
Under "Passwords and forms," click Manage Autofill settings.
Point to the entry and click Delete Close.
Note: If an entry is saved in Google Payments, you can't delete it from Autofill. To change or delete the entry, click Edit and sign in. Then, make your changes in Google Payments.