Firefox ESR (formerly rebranded as PureBrowser) should not by default offer a search engine that tracks users.
Historically (before 2019) PureBrowser used Google known to track its users.
in June 2019 PureBrowser used DuckDuckGo which is believed to not track its users.
When rebranded PureBrowser was abandoned and Debian Firefox ESR reintroduced, the default search engine switched back to being Google.
NB! This issue is not about server-executed code being non-free (see T110) nor about it serving non-free JavaScript (see T609).