Hrmm...a few more observations here. This is looking a little bit like a bug/issue with 1.3b15.
On a whim, I tried to apply the 1.3b7 firmware to downgrade from 1.3b15. The firmware uploaded successfully and the m0n0wall said it was rebooting to apply the firmware.
However, after the reboot it was still at 1.3b15 so I suspect there is something in the code that prevents it from downgrading.
So it's interesting that the 1.3b7 image will load via the webgui, but the 1.3 image will not.
Also, I noticed that after attempting to load the 1.3 image onto 1.3b15 and getting the page reset error in my browser, m0n0wall goes into 99% memory utilization and stays pegged there until I reboot the m0n0wall.