User Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.14; rv:66.0) Gecko/20100101 Firefox/66.0

Steps to reproduce:

1. Use History > Clear Recent History... to clear "Browsing & Download History", "Form & Search History", and "Cache".

2. Open a new blank tab or window, put my cursor into the location bar, and start typing something. Two examples are just the letter "t" or just the letter "p".

Actual results:

I'm presented with some legitimate suggestions from my bookmarks, but the primary suggestions in these cases are some sort of erroneous possibly internal identifiers or fragments of URLs/URIs, composed of what appear to be query strings.

When I type a "t", the first suggestion is presented as "place:type=6&sort=14&maxresults=10".

When I type a "p", the first suggestion is presented as "parent=menu________&parent=unfiled_____&parent=toolbar_____&querytype=1&sort=12&maxresults=10&excludequeries=1".

Pressing Shift-backspace or Shift-delete does remove the suggestion, but only until the next time I start typing the same letter or string.

Expected results:

The primary suggestions presented as "place:something" are not valid URLs, even when prepended with "http://" or "https://" rather than "place:"; none of them appears in my bookmarks; and I've just cleared my history. I don't know if they represent some kind of query format that's used in the internals of Firefox itself, or what they could be. But they should not be offered.

If there is some legitimate reason that such a suggestion would be offered, pressing Shift-backspace or Shift-delete with the suggestion selected should permanently remove it. Alternatively, there should be some way to clear out whatever data source is being used to generate these suggestions.

Other notes: I've been having this problem for a long time, not just in version 66, so this is not a new issue for me. The only extension I have installed is the 1Password extension, but the issue persists even if I disable that.

I'm attaching a screen shot illustrating the "t" example.