I don't think Firefox on Android supports this. On Desktop it's quite useful for when you want to search a site you don't have a shortcut for. Guessing the shortcut has usually worked for me.

Also, when your search didn't give you what you were looking for, you can just prepend e.g. !g to search on Google, which for me is slightly faster than to copy the query, go to the address bar, enter "g" and then paste.