Browser URLs Upvoting

You already can, in a roundabout way.

If there's a site I want to be the first result after I've just typed just a single letter in the address bar, I type as little of the sites name as I can before it appears in autocomplete. Then I click the site, close the tab, and start again. After doing this a few times, Firefox usually gets that this is the site I want to go to after I've typed g or f or whatever, and puts it as the first result.