PIX Firewall is not available with a Gigabit Ethernet LX optical interface. There is probably just not much of a market out there for one.
Gigabit Ethernet SX over monomode (single mode) fiber is not supported. It may work over a short distance, such as a 2-meter patch cable length, but I wouldn't want to rely on it in a production environment.
To start with, the diameter of the monomode fiber is so small (8-9um vs. 50-62.5um for multimode) that it reduces the launch aperture area by more than 96%. On top of that, I believe the transmitter on SX optics is offset from the optical axis of the fiber, for optimum performance with multimode fiber; the offset is probably enough that most if not all of the optical signal doesn't even hit the available launch window of the monomode fiber. Consider yourself lucky if any usable signal is received at the other end.
You're better off terminating the monomode fiber run on optics that it is intended for. A Gigabit Ethernet LX (single mode) to SX (multimode) media converter would work. Or, if you have two available GBIC slots in a switch, use an SX and an LX/LH GBIC and put those two ports in a unique VLAN, and let the switch be your media converter. Cost of the GBICs is probably about the same as the media converter, you just need the two GBIC slots to make it work.
Hope this helps.
