HTTPS has no relation to Firefox or any other browser. Only the thing it matters:
There are two different settings involved:
Address Bar Autofill
If you have used HTTPS on a site before, Firefox’s address bar autofill feature will prefer the HTTPS address. If you don’t mind selecting from the drop-down instead, or simply typing the whole address, you can turn off the address bar autofill. Here’s how:
(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 autofill and pause while the list is filtered
(3) Double-click browser.urlbar.autoFill to toggle it from true to false.
Address Bar Search
Regarding address bar search, you can change the URL used by the “keyword” service. Please see this thread for links: https://support.mozilla.org/en-US/questions/955363