keyword.enabled set to false worked for me.
With the default setting (true) - and using the example from that link - entering localhst in the address bar, sent a search request to my default search engine for localhst and that of course gives no useful results.
Changing that setting to false - entering localhst resulted in a complaint that www.localhst.com does not exist as a domain (and I can see from my DNS logs that it did not try any other variations).
Important to note though that it is a case of choosing your poison. If the entered word were sensitive information, you are just choosing whether to share it with a search engine or with the DNS.
Finally, setting browser.fixup.alternate.prefix to the empty string, changes the complaint to localhst.com (and that is what it looked up) and then further setting browser.fixup.alternate.suffix to the empty string, changes the complaint to localhst (and nothing was sent to the internet, I think, because the domain will then be resolved locally by my DNS server, and fail because there is no host on my network with that name - this might technically be a bug in my config but in practice it solves the problem).