As x.15a2 says, it could be that your browsing history is being saved (default behaviour). Other things could be search suggestions, which can be turned off in Firefox in Preferences -> Search.

Also, it might help to switch off search queries displaying in URLs by going to DuckDuckGo advanced settings -> Privacy -> Requests shown in url (turn off and save). Note that this only affects searches made from the DuckDuckGo website - not searches from the browser address bar or search bar.
posted by tagawa Staff3 years and 2 months ago Link