Next time use more letters, like; sudoku. Dennis, delphi, and so on.

Obviously more letters will narrow the criteria but that does not answer my question. Why is it -necessary- to type additional letters to exclude rarely visited retail sites. Isn't frequency of visits or most recently visited a more useful sort criteria?

If you like math... check out this article: The Places frecency algorithm | MDN.

To see whether you have any non-default weightings, or to adjust them, you can access them as follows:

(1) In a new tab, type or paste about:config in the address bar and press Enter. Click the button promising to be careful.

(2) In the filter box, type or paste frec and pause while the list is filtered

Any modified values will be bolded. If things go terribly wrong, right-click>Reset should get you back to default.

You may also be interested in this you can for instance just add an asterisk when you type su or de and the results are then restricted to bookmrke addresses.

• Address bar autocomplete suggestions in Firefox_changing-results-on-the-fly

You can consider to bookmark that site and add a keyword to that bookmark if you visit the site that often.
You can do that in the Bookmarks Manager (Library) if you click the More button at the bottom of the right pane after selecting that bookmark.