Please read again. I said:

"Firefox doesn’t actually issue a warning, which would have made my job tracking this issue down much easier. Moreover, functions declared within a block before being called work just fine, so it's not that function declarations can't by used as statements so much as SpiderMonkey won't bother to hoist them before executing any other code in that block."

As I said, the grammar in the ECMA-262 spec only allows Statements within Blocks, and a FunctionDeclaration isn't a Statement.

None of the browsers adhere to the spec, but I suspect they're just trying to play nicely with legacy content. Firefox just happens to play differently to all the other browsers.