Why does my typing get delayed sometimes on my browser?

I’ve noticed this issue happens when I’m on my browser (Firefox) once and a while and I’m typing blog posts on tumblr or replying to something on Facebook or even typing here on Fluther. Just basically any website where I can enter text. It does this thing where it seems as though it can’t keep up with how fast I type and I have to backspace and put some letters together or delete words. It’s really irritating when I’m in a rush. Never happens on Word or Notepad or any other application other than my browser. I heard a tip to check Task Manager and look at any other process that’s going on while I’m typing that’s taking up CPU usage, but the only thing that shows up is Firefox, obviously, and it takes about 20% or so when I start typing. That kind of got me nowhere, except how much it takes in usage.It doesn’t happen all the time, but once and a while. I can’t really figure out why it happens when it does. It doesn’t seem like a laptop heating issue; mine hardly heats up that badly.