Just Search Qwant od Naresh

The official Qwant extension offers too many features for some users who simply want to use it as a search engine. This extension is extremely simple, only adding Qwant as the default search engine, without all the extra fluff of the official extension.

Check out the source code, it literally doesn't get simpler than this:
https://github.com/naresh97/Just-Search-Qwant/