I couldn't find this info anywhere in this thread, so I thought I should mention that it's possible to update the firmware using a Mac with a Mini-Toslink connection (i.e. most Macs made after 2006), no CD player required.
If you don't have a Mini-Toslink to Toslink adapter or cable, you can get an adapter from Monoprice for less than a buck. Do a search on Monoprice for "2671", that will bring up the appropriate product ID page.
Plug it into your Mac's headphone jack, and then connect an optical cable from the adapter to the "Optical AV 1" port.
If you then go to the "Output" tab of the "Sound" section in "System Preferences", the output should show "Optical digital-out port".
NOTE: CLOSE ALL YOUR APPS AND TURN YOUR WIFI/ETHERNET OFF BEFORE YOU CONTINUE. If your Mac plays any other sounds from notifications or iMessages or whatnot, it will probably screw up the firmware update.
Put your receiver into the software update mode by unplugging your receiver, and then plug it back in while holding the "TUNING >>" button; if you did it correctly, the display will say "CDDA UPGRADER".
Now you just need to play the audio file. I wasn't able to get the update to work with VLC or QuickView. Luckily OSX comes with a CLI app called "alplay" that does the job:
Open a Terminal window, and run
afplay -d -q 1 ~/Downloads/RXV667_767_FIRMWARE_V211.wav
...obviously replacing the path with the correct path to your firmware file.
With any luck, your receiver's display will change to indicate that it's updating the firmware. Let 'er go, it'll take about 15 minutes.
Also, I should mention that I was able to update my HTR-6063 with the RX-V667 firmware, since those models are identical, even though Yamaha hasn't officially released any firmware updates for the HTR-6063!