People are able to place firebases where they hang halfway off an edge, and also where they sit halfway inside walls. There should be a check for a certain radius of clear, relatively flat space
So tat teh kiddiez can finnaleh shewt teh fireybasez. But on a more serious note. I believe its a glitch and not intended. And sometimes there firebases are so well embedded into the wall that you can't even hit that lil bugger.