I notice that wikipedia/manifest.json is the only enabled search provider that neither specifies a search_url_get_params/search_url_post_params, nor hardcodes the params in the search_url. (They are hardcoded in search_form. I guess Firefox does not use this anymore?)
Okay, here's the one-liner patch. [Edit: Replaced by a prettier v2 that also includes an explanation.] I verified that it works by rezipping the updated Wikipedia manifest.json into tor-browser_en-US/Browser/browser/omni.ja.
Thanks for the patch. I cherry-picked it (with slightly adapted commit message) to tor-browser-68.2.0esr-9.5-1 (commit 9c6f3617) and tor-browser-68.2.0esr-9.0-1 (commit d9bdd00c).
Trac: Status: needs_review to closed Resolution: N/Ato fixed