I finally got it to work after a bit of tweaking the path to the exe file for MPC-HC, and it launched and played a video from the YouTube website page, and it played a few seconds of video with no sound, then stopped and played the audio only stream with a black screen, and then it stopped working. No further attempts to play any video worked. I'm on Windows 7 32-bit in Firefox on an old Lenovo Thinkpad Z61p and using a Broadcom BCM970015 video decoding card. Computer fully updated and working very well otherwise.

Dear BELAV, please read reviews about your add-on, there are plenty of complains about your product non functionality!
I added your extension to my FF because I am big fan of MPC-HC player. Unfortunately, your extension does not work in my FF 68 on 64-bit Windows.
Kindly waiting for progress, as _you_seem_to_be_the_only_one_ who make a try to build integration for MPC-HC

The default path didn't work for me, and I had to manually put the path in quotes to get it to work, furthermore the player hangs when you try and exist it before a video from YouTube has finished buffering causing you to have to force close the external player.

Не работает в FF ESR по пути %ProgramFiles(x86)%\K-LiteCodecPack\MPC-HC64\mpc-hc64.exe
http://i100.fastpic.ru/big/2017/1216/3f/87109d82dcb2a09bec3812b7a44a5d3f.png