Purebrowser could use DuckDuckGo (not Yahoo or Google) as default search engine. (As of 2019, Purebrowser defaults to DDG)
In the future, Purebrowser could use another centralized but more ethical service (when available), or a decentralized tool like yazy (when packaged in Debian).
NB! This issue is not about server-executed code being non-free (see T110) nor about it serving non-free JavaScript (see T609).