Hi,

The list is dependent on the setting in Tools (Alt + T) > Options > Privacy > Location Bar. Options > Privacy. So for eg. if both bookmarks and history is selected there, visited and bookmarked websites will appear in the location bar drop-down list. From the drop-down list you can delete the visited sites (history) entries by using the up/down arrow keys to highlight the entry and Del on the keyboard. To delete the bookmarked entries you may have to delete the bookmark itself. Bookmarking sites/tagging them would add them to the list.

Location Bar

You can only remove entries from the location bar drop down list that aren't bookmarks by pressing the Delete key on an highlighted entry.
Otherwise such a bookmarked link will reappear if you revisit the site or if there is still a visit count.
Bookmarks have a yellow star at the far right end.

You can't add entries manually. Firefox creates that list automatically based on bonus points (frecency) that are given based on how many times you have visited a link.

Thank you both for your answers. You solved my problem.