As you know, but seems to need restating, the vast bulk of what actually comprises the Custom "Firmware" downloads to the hard drive after the tweaks to the firmware residing in flash are applied by USB installation. I'm certainly not underestimating the effort required to come up with those tweaks in the first place, but nonetheless they are a small percentage of the code in flash, and an even smaller percentage of custom code overall - which is why I was always inclined to call it software rather than firmware.
But we don't know what was in the OP's mind - was he talking purely standard firmware, or CF? This is in the vanilla section of the forum. If you wish to inspect the version number for the CF, it is displayed on the front panel during boot, or at the bottom right of the WebIF home page.