Oh, okay, didn't know that :)
I just think it sounds a bit strange that the operating system wants to open an app using the browser (but this could very well be perfectly normal, I'm not a Mac user). Is there something equivalent to the Windows "Right click -> Open with..." action on Mac? Maybe at some point something got mixed up and apps like this are supposed to be opened using a different program? Just shooting in the dark here, the most likely explanation is still a problem with the game engine Unity.