The title is a quote from the second sentence in the article. The original title of the post is about the immediate change, but Mozilla is still planning to deprecate OpenSearch.
Firefox definitely has supported OpenSearch. When you arrive at a page that has an OpenSearch description document, there is a button in the address bar triple-dot menu to add the site's search engine to firefox. On mobile, you could long press on an input field to do the same.
Even though I'll miss it myself as a power user I think this is actually fine so long as there's a common standard for integrating third-party engines.
Opensearch came about when search was in its nasency and people envisioned all sorts of complex needs for it. 90%+ of opensearch schemas just end up being something like "searchengine.org/?q=%s" in practice.
The main thing we'll lose out in the simpler pattern is autocomplete, which you could specify via "<Url type="application/x-suggestions+json"
What the article doesn't make clear and I wish it did was what Firefox's future of supporting third party search engines looks like. I just came back to firefox after 8 years away, and it seems from some of the UI changes like the "This time search with" feature that they do want to encourage a diversity of search engines overall.
Firefox definitely has supported OpenSearch. When you arrive at a page that has an OpenSearch description document, there is a button in the address bar triple-dot menu to add the site's search engine to firefox. On mobile, you could long press on an input field to do the same.