In recent versions of Firefox, single words are looked up via DNS when you enter
them in the URL bar. If the word resolves, the browser "helpfully" indicates
that there's a page at http://<word> to visit.
Weird feature, though when taken at face value I can see its usefulness with a combined search and URL bar. Ideally you'd have completely different semantics for searching and (direct) browsing, but I don't think something like this is forthcoming. If anything I'd expect direct browsing to be ever more discouraged. Nowadays search engines, not browsers, are the gateway to the internet.
In any case, turns out this feature also has interesting side-effects with (browser-based) DoH turned on. If you ever need to turn it off, Firefox 78 added the following about:config switch:
browser.urlbar.dnsResolveSingleWordsAfterSearch
Not that I think this is a great solution. DoH should be done on a system-wide level (with local queries sent to your router or resolved through mDNS) and there should be a way of trusting your router's DNS to not forward local queries to your ISP.