edit: or apparently on Firefox you can also just open https://noai.duckduckgo.com, right click the search text box, and click add search engine. lol

previous:
how to switch to noai.duckduckgo.com on firefox without installing any extensions or plugins:

• open settings > search
• scroll down to search shortcuts and click add
• fill out according to the image below (text in alt text)
• click add engine
• scroll back up and select "DuckDuckGo No AI" under "default search engine"