This is a limitation of how AO works and it's the same in every other renderer. Some renderers treat all objects as 'opaque' for AO, no matter what the transparency, others treat objects with even a hint of tranparency as 'invisible'. Firefly happens to do the latter. There's no way around this as AO is calculated purely by 'touching' objects around each pixel and not calling their shaders.

To get degrees of transparency, what is needed is an advanced version of AO, called Final Gathering that Firefly doesn't support (and is a lot slower then AO besides)