The bangs work any place in the string. They don't have to be a prefix.

In Firefox you have a dedicated search bar (ctrl-k) which remembers your search across locations. So you search for "heisenbug oracle ipv6" press enter and are not immediately happy about the precision of your search.

So you press ctrl-k again and append "!so". Bang, your previous search is now applied to stack overflow and you have your answer straight at spot 1.

It's a very good flow. It will never work in retarded browsers who insist on removing the search bar though (like Chrome, Safari, IE).

In a deeply misguided act of Chromeism, Firefox was considering going in that direction too, but the outrage in the userbase hopefully caused them to never venture that line of thought again.