Switching between search engines in Firefox search box quickly?
Ctrl+Up/Down arrow switches to next/previous search engine in the list.
If you want the search box to always go back to the default after a non-default search, install and customize the SearchLoad Options extension.
Alternatively, you may be looking for Second Search.
If you really want to be able to use a specific engine by typing its name, you want something like Obnibar or even Firefox's built-in keyword search feature. There are many other extensions for variations on keyword searching. Browse the Firefox Add-ons website.