More than 560 KB for a search plugin is a joke. This adds a lot of unnecessary functionality: A toolbar icon (who needs that?), context menu entries (anyone using that?), access to the tracking protection setting that you can also change in the normal prefs... and even with all of that, I don't see why it needs far over 500 KB, it should need less than 50 KB.

Here's how you get the same functionality as OpenSearch plugin w/o that stupid extension:
Make sure no Qwant extension is installed, customize your toolbar, make sure the search field is visible, open Qwant search in your browser, the search bar shows a little green + button on the magnifying glass, click it, it offers you to add Qwant as a search engine, select that, and you are done (all it installs is a 10 KB XML file!!!). Now you can remove the search field from your toolbar again if you don't like to have one.

PS: This is no rating of the search engine Qwant, this is just a rating for this extension! I like the search engine but I dislike a bloated extension for something as simple as a custom search.