Yes, when there is one word, Firefox will try it with www. at the beginning and .com at the end (depending on your location). If there is no such domain, you get the error from your DNS provider.

To prevent that, turn off the "fixup" feature here:

(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 fixup and pause while the list is filtered

(3) Double-click browser.fixup.alternate.enabled to flip it to false. That should take effect immediately with your next search, and you can close this tab.

In this thread, the user had to switch it to true, then back to false to reset it: Won't google search when one word is written in address bar.

You may have an ISP that automatically redirects you to a search page if a DNS look up fails.
That takes the control from Firefox to do the normal keyword search like a Google I'm Lucky search.
In such cases you need to contact your ISP and ask for a DNS server to opt-out of that feature.
You can also check if there is a link on the search results page to go to your ISP and opt-out.