You don’t get it with the default omnibar, however.
We also refer to rel="search" on the reference page, and point to this documentation.
I think we should move the page out of AddOns (maybe as a top-level page), and start making it a generic document for the OpenSearch standard. It seems like a useful open web technology that didn’t quite make it, but still could be incredibly useful for sites like developer.mozilla.org, stackoverflow.com, and intranet sites.