1. Press Alt or F10 to see the menu bar
2. Click on Tools >> Options >> Privacy
3. At bottom of the window, in the session Location bar, change the option When using the location bar, suggest: to Nothing

Also:

• Remove websites from the address bar suggestions

I like the autocomplete feature so I have Bookmarks checked in the Location bar but I want to clear items (sites) from the Location bar history.

I followed Clearing a single item from the Location bar history from

https://support.mozilla.org/en-US/kb/remove-websites-awesome-bar-suggestions

but I can’t clear them.

Try do this in safe mode.

Probably your places.sqlite file is corrupted try see Locked or damaged places.sqlite