Some Firefox users noticed playback issues on YouTube for several months. These affected high resolution videos only, from 1080p and up. To make matters worse, no clear pattern could be identified.
Some videos played fine, others would stop abruptly when they ran out of buffer.
Any piracy costing money is also really really likely to be shut down relatively soon, making you need to change your setup.
Been using it for 2 years, no issues so far.