• Click the (empty) input field on the web page to open the drop down list
• Highlight an entry in the drop down list
• Press the Delete key (on Mac: Shift+Delete) to remove it.

• http://kb.mozillazine.org/Deleting_autocomplete_entries

Make sure that you do not get confused by Search Suggestions:

• https://support.mozilla.com/kb/Search+suggestions

If you mean the location bar then see:

• https://support.mozilla.com/kb/Clearing+Location+bar+history
• https://support.mozilla.com/kb/Cannot+clear+Location+bar+history

Entries in the location bar drop down list with a yellow (blue on Mac) star at the right end are bookmarks and those entries can't be removed from the location bar drop down list with the Delete key.
You can remove such a bookmarked item that shows in the list if you open that url in a tab and click the yellow star in the location bar.
This will open the "Edit This Bookmark" dialog and you can click the Remove button to remove the bookmark if you want to remove such a bookmarked entry.

thanks a million thought I was crazy

This does not clear out the autocomplete. As soon as you open Firefox again all the recommended entries you just deleted are offered again. I have deleted cookies, history over and over and over and it will not delete the autocomplete suggestions.