FirstClass has a super-secret debug console hidden inside of it. It seems like you should be able to access it from some sort of key combo on startup or something similar (have to look closer at the code), but right now, the easiest way to access it is to patch the client binary and change the check for the console from a jnz to a jz.
The packet debugger and the debug menu seems to have existed in some form since at least FirstClass 8.
note that this only applies to the Mac version 12.122 it will probably not work on another version
also this breaks using keyboard shortcuts and the arrow keys so do it on a copy of the application
To do this, use a hex editor to find the sequence 0F 85 35 00 00 00 66 8B and replace it with 0F 84 35 00 00 00 66 8B (the 85 becomes 84). Then, run the binary of the application (not the application, but the binary inside of the .app. You want to see the console output), log in, and in the view where you see the icons (like your mailbox and trash and etc), press the ?, and you will see this menu:
p - Packet Debug
d - dump the packets currently in the buffer
l - open console, not useful if you can see this, but for next time
a - lookup an address to get the symbol closest to it
t - do a not very useful traceback
s - Toggle Steve's super mode
b - Toggle save message bodies and docedit to download
The most useful option is p, which opens a packet debugging window that shows you what packets are sent and received for different actions.
Another useful binary patch (FOR FIRSTCLASS 16.103 ON OS X) is changing addresses 0x864F-0x8652 (which should be all 0x00) to be 0xFF and address 0x8653 to 0x7F. This is where in the code _gFCPDebugLevel is set, and will change it from being zero to the maximum signed 32-bit integer, enabling all FCP debug logs.