Here is another suggested solution from another solver:
"I had the same problem. Opening Firefox in Safe mode cured it, so I knew it had to be a setting or plugin of some sort....I did all the recommended fixes and nothing changed the typing being real slow and behind when running in normal mode.
What fixed it for me is turning off the hardware acceleration option in Firefox. Here's how: Click on Firefox->Options->Advanced->General tab. Then uncheck the checkbox "Use hardware acceleration when available". Click OK. Hope it works for you. Good luck!"