You would have to make the browser engine run through the Tor proxy (socks5://127.0.0.1:9050), including DNS requests to resolve onions. But why no one forked Tor? It's probably because Firefox and its Gecko browser engine aren't dead yet, but it might be in the future, so it's probably a good idea to use Pale Moon as a replacement, especially with the Proxy Privacy Ruler, which allows for applying the proxy only for private windows and/or certain domains. But they'll probably not do that and just accept to use Chromium and its Blink browser engine (I mean, Pale Moon is bad... but it's still better than what Chromium is trying to do).