http://esptimer/ is not a proper web address. All addresses I have seen have <dot> something at the end.

and when I try http://www.esptimer.com/ I get:

Server not found Firefox can’t find the server at www.esptimer.com

^: note that this is about a local server and not internet.

You can look at these prefs on the about:config page.

• browser.fixup.dns_first_for_single_words
• browser.urlbar.dnsResolveSingleWordsAfterSearch [0:never resolve;1:use heuristics(*);2:always resolve]
• https://searchfox.org/mozilla-release/source/browser/app/profile/firefox.js#348

See also:

• browser.fixup.domainwhitelist.*
• https://msujaws.wordpress.com/2014/08/01/faster-and-snappier-searches-now-in-firefox-aurora/

Is it possible your Firefox is using DNS over HTTPS in strict mode? If so, you may need to create exceptions for your local host names. You could take a look here:

(1) In a new tab, type or paste about:config in the address bar and press Enter/Return. Click the button accepting the risk.

(2) In the search box in the page, type or paste trr and pause while the list is filtered

If the network.trr.mode preference is bolded and has a value of 2 or 3 then DoH is enabled. The difference is:

• Mode 2: if your trusted resolver doesn't know the address, Firefox should check local DNS
• Mode 3: if your trusted resolver doesn't know the address, Firefox should assume it doesn't exist

If you are set to Mode 3, you could try Mode 2 instead.

Alternately, or to save a lookup, you could try this:

(3) Double-click the network.trr.excluded-domains preference to display an editing field:

• If it's blank, type or paste esptimer
• If you already have some host names listed, ad a comma at the end and then type or paste esptimer

Either press Enter or click the blue check mark button to save the change.