The "legacy" search bar doesn't support a lot of feature of the address bar, and that's also why in general we suggest users to stick to the address bar, that is more than able to execute any kind of search, and also leep separation between urls and searches, with appropriate settings. In the future we hope to be able to share the code and then this will be feasible, but for now there's no plan to port new features to the old search bar code.
Bug 1891425 Comment 2 Edit History
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The "legacy" search bar doesn't support a lot of feature of the address bar, and that's also why in general we suggest users to stick to the address bar, that is more than able to execute any kind of search, and also keep separation between urls and searches, with appropriate settings. In the future we hope to be able to share the code and then this will be feasible, but for now there's no plan to port new features to the old search bar code.