In the dev tools settings there is a checkbox "Disable HTTP Cache (when toolbox is open)" which name seems to imply that this only applies to the resources that make up the page, not to DNS lookups (about:config name is devtools.cache.disabled, and it defaults to false).
I investigated and found that Firefox's in-memory DNS cache can be manually cleared by clicking a button in about:networking. To be fair Chrome also has a similar cache and method for clearing it. See: https://www.makeuseof.com/chrome-edge-firefox-safari-opera-b...