FireWire is (was) an interesting beast. It was essentially a data communications protocol, but in the late 90s, video industry adopted it as their standard protocol for streaming standard-definition digital video (DV), with a standardised data format. Apple, the designer of FireWire, provided support for this data format inside Mac OS, which allowed Macintosh computers to natively understand DV stream coming via FireWire port. During the std-def years of DV, this worked beautifully; you could hook up any standard DV camcorder via firewire to the Mac, and the computer would automatically recognise it as an audio/video input device (essentially, a webcam). Any application that provided video capturing services (QuickTime, Skype, iChat AV, iMovie, Premiere, etc) could see it as an AV source and use it.
With HD, the digital A/V streaming protocol was built on HDMI. However, HDMI is NOT a standard data communication protocol used by computers in a way that FireWire was used. HDMI flows only in one direction, so a HDMI port on a Mac is strictly an output port. Modern Macs now have USB and Thunderbolt data ports, but neither of these is actually compatible with HDMI. In other words, you can't simply connect HDMI-to-Thunderbolt cable and capture live A/V stream from that camcorder. While there IS a thunderbolt-to-HDMI cable, its only purpose is to take the A/V component on the thunderbold port and convert it into HDMI for output to TV monitors. This is completely unrelated to the data traffic on Thunderbolt.
So, today, the only way to take a HDMI signal from a camcorder and capture it on a Mac is buy purchasing a $100+ HDMI-to-USB/USB3/Thunderbold (whichever target connector you need) capturing device, which will take that HDMI signal and convert it into a proper data stream that the computer can understand and capture.
People here have recommended Blackmagic Intensity Shuttle, Magawell, AJA, etc.
