Ok, I've figured it out. The Shift+Delete combination only works for search results (e.g. the results displayed when you press space or type part of the url that are in your history).

To manage the top sites (which Firefox 78 displays on address bar click or down arrow if enabled in the preferences [1]), you need to: 1) add the top sites on the home page, 2) hover over the page you want to remove, 3) click the "..." button in the top right ("Open Menu"), 4) click "unpin" if you don't want it to be kept there, but stil calculated in the ranking, 5) click "dismiss" if you don't want it displayed.

[1] This is a change in Firefox 78, as when pressing the down arrow key Firefox 77 displayed what Firefox 78 displays when pressing the space bar.

It doesn't work for me either. It comes up as a solution whenever I try to solve this particular problem but it doesn't work for some reason.

try searching for the item in your History sidebar, then right-click -> Forget about this site.

I've done this and the OPs suggestion, and the incorrect autocomplete persists. In my case, I went to a random person's blog that starts with the same two letters as a website I use frequently.

I've given that random person's blog thousands of hits over the last year...

If it is in your top sites, you need to add that widget to the home page to get access to the menu to dismiss the result. (See my reply to the grandparent post for details.)

I don't know of an easier way to do this.