My computer has been running slow and “herky jerky” the last few days. I’m also getting a “high disk usage from Firefox” warning. But I can’t find out why. I’ve disabled all my add ons and plug ins from Firefox. I haven’t downloaded any other programs and such that could take up memory/disk space.
When you post an answer please keep in mind I’m a total mental when it comes to computers. Retard English, please. 
How full is your hard disk? Go to Explorer (not IE) and right-click on the C: drive and choose “Properties”, you should get a box pop-up that says how much of the drive is used and how much is free. You need to have at least around 10% of the disk capacity free for the programs to run.
I have no idea if this is your problem, but it’s what I would check first.
excavating (for a mind)
Just guessing, but it sounds like Firefox might be warning you about how much is in the cache? Try clearing your cache, and seeing if it the message goes away.
You can configure how much Firefox caches. The exact place varies, but try going to Tools -> Options -> Advanced -> Network / Offline Storage.
Let me guess, a Norton product is the source of this message? If so you can probably ignore it (still, check that you have sufficient disk space as described in post 2).
It is possible that Norton is the reason your computer is running sluggishly too (although less likely if you have had Norton all along and only recently had a degradation in performance).
I’d remove it and replace it with many of the superior completely free products (Microsoft Security Essentials is my favorite). See how your system performs for a few days.
AVG also frequently will say something is experiencing “high memory usage”. You can safely ignore this. It’s just letting you know whatever you’re doing is using more memory than it feels is necessary for browsing.
Try running procmon and other SysInternals tools (you can download directly from Microsoft). Some of these tools can help you identify what’s helping use system resources, and are fun to play with, if you want to play “geek for a week.” It won’t take you a week, though, I just used that because it rhymed.
Yes, if you are running AVG and you keep getting that annoying message about memory usage on Firefox you can turn it off. Bring up the AVG interface, go to Tools/Advanced settings/Appearance/System Tray Notifications and untick the box next to Display AVG Advisor performance notifications. Bingo, no more message.