If something is stable, then you do not need to spend efforts on it and develop the driver further.
But as there is always a chance that something changes in terms of kernel structures and what not, all driver need maintenance, so that it goes error-free through the compiler and doesn't produce wrong code. As a result of this it needs to be tested.
These efforts are a point of discussion wheter or not to spend efforts for Firewire.
For Apple the meaning of firewire is IMHO low. The FW support by Thunderbold is IMHO only based on the fact, that thunderbolt simply tunnels the firewire protocol through the quicker transfer medium.
If Thunderbolt wouldn't be by design such a tunnel for other protocols/data streams then I would have doubts whether support for Firewire would have been re-written by apple for thunderbolt.
If you need safety and are shortly before an invest I would nowadays always try to buy a product which is based on both USB and Firewire. Alone for the reason that its never 100% predictable, if the one or other runs better on your system, at least on PC. How it is with Apple I can't tell by heart, but from what people tell it shall run very good there.
But while thinking about this .. has Firewire still its asynchronous nature when being sent through the "Thunderbolt tunnel" ?
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