Whenever I play videos on Firefox on any website, the videos start lagging hard. The audio will keep playing fine, but the video acts like it keeps buffering, even though it's fully buffered. This has been intermittently happening for about 2 years. It started happening again today after about 6 months of it working fine, and that was the straw that broke the camels back. So, I've factory reset Firefox, I've ensured it's up-to-date, I've checked Java and Flash, both are up to date. My graphics card drivers and OS (Mac OS X 10.11) are up to date. I have a GeForce 750M, and quad-core i7 with 16GB of RAM, so it's not a hardware issue. I have a 75 Mbps connection, so it's not an internet issue. And I can play videos just fine in other browsers, but I'm a programmer and other browsers don't have some of the addons I use. So I'd like to keep using Firefox. Currently I'm using a completely vanilla version of Firefox and it's still giving me the same issues. Just typing this is occasionally causing Firefox to lag. I've tried everything I can think of to resolve the issue. I'm out of ideas. I'm hoping someone here can help. Thanks in advance.