Hi Serge, unfortunately, the @ feature is hard-coded at the moment. It lists only the built-in search engines, and lists them even if you unchecked or removed them in the One-Click Search Engines box on the Options page. We can hope this feature becomes more flexible/adaptive in future versions.

For now, you can create a keyword for the search engine. For example, let's say you prefer to use Startpage. If you double-click the blank space In the keyword column on the Options page, you can give it a brief code (for example, sp). Then in the address bar, when you start with the keyword, Firefox will immediately show that it plans to use that site. I have attached a couple screenshots illustrating this.