I don't know about Peg2, but I recently did some debug via FireWire between two different Mac setups. First between two PowerMac G5 machines and then between Mac mini and Powerbook. It worked in similar way in both cases.
As the FireWire support has been enabled in the boot.img since 3.12, I had to disable it on the machine I run ohci1394_mon, because it doesn't seem to work if Helios is running. To disable it I gave "fwdebug" (or shorter "fd") argument to the boot.img. If FireWire support isn't enabled in boot.img on Pegasos2 but only on Macs, then this step can be skipped. But in any case the FireWire support must be turned on on the "source" machine you want to get the log from, and turned off on the "destination" machine you want to get the log to.
I didn't get ohci1394_mon to work by any other way but in the "interactive mode". So I started it by "ohci1394_mon ia", and then used the "dl" command to view the log in real time, or "dl ram:fwlog.txt" to save the log to a file. Don't use plain "dl" before "dl file" if you want to get the full log, because "dl" clears it to the point it displays it.
I needed to get the debug at boot time, so I also gave "ed=init" argument to the boot.img. Not related to this case, but I'll document it in this post if needed in the future ;)
Many thanks! It works.
Pegasos2 don't need fwdebug argument. The thing is INTERACTIVE mode on iMAC, and not SERVERMODE which I tried before.
AmigaOS: Pegasos II, Sam440ep-flex, Micro A1-C, AmigaOneX1000
MorphOS: Pegasos II, Powerbook G4, iMac G5, Powermac G5 Quad