I recently revisited the topic of the Vivitar 285HV and fireflies as another cave photographer had asked about slaves that would trigger them properly. I found that all my Wein and Gibson slaves worked fine on them, but this caver also had been unable to trigger them with fireflies.
In researching the Vivitar I found one crucial difference between the 285 and the newer 285HV, and that's in the "trigger voltage," or the voltage across the hotshoe contacts. In the older ones it can be very high, on the order of a couple hundred volts. The HV model, however, is designed to have a very low voltage across the shoes in order to prevent damage to digital cameras, which purportedly can be fried by high trigger voltages. I don't know all the electronics involved in the Firefly but suspect that it is this lower trigger voltage that makes the difference. It would be interesting to see if other flashes that don't work with the firefly also have such low trigger voltages. Of course, as Bryan found, you can take the thing off between shots, but what a pain, especially if its sitting out on a rock somewhere that you had to do a delicate climb to place it...