Abrowser: control of usage of search engines

I always add a separate box to search something using search engines and I want anything I type in the address bar to not ever go to a search engine.

In the search preferences, there are multiple options about "suggestions" from the address bar, among which one is about search engines. Whether it is ticked or not apparently has no effect on whether or not something typed in the address bar that is not a URL goes to the default search engine or not.

What I found out is that, if I set keyword.enabled to false in about:config, I will just get an arror if I have the cursor in the address bar, I type enter and what is there cannot be resolved as hostname, which is what I expected. If I set if back to true, again it goes to the default search engine.

Besides, whether the "search engines" box below "address bar" in the settings is ticked or not, switching keyword.enabled to true or false completely controls the use of the default search engine in the address bar. Does anyone know what this tickbox is actually controlling?