I used the GUI to try to upgrade from 1.3b16 to 1.3b18 but nothing seems to happen. m0n0wall looks like it is going to install the update and then reboots but no actual upgrade. I have tried in IE and Chrome, no difference. I have tried to update from two different computers also.
I'm having the exact same problem going from b16 to b18. The md5 of my firmware image is correct. I tried from Safari, too, just to be different than the OP. I'm upgrading a PC Engines WRAP with a 32MB CF card. I've had no problems in the past with firmware upgrades.