browser freezing

It happens in both of my browsers, Firefox and Opera more in Firefox than Opera. So I assume it might be OS related. I will be on a page reading an article, not using the mouse at all, so after about 7 or 8 minutes, the screen will start to darken from the top down, the mouse pointer is either stuck in one spot, or if I can move it at all, it won't close any tabs or even the browser. If I click if click on the close browser button frequently, a FORCE QUIT window will pop up. If I do nothing, eventually a STOP SCRIPT window will pop up. This whole business is very annoying.
Do you all have a solution?

I'm afraid not - you could see if running in a different DE might help? e.g., Cairo-Dock DE/Cinammon - see if disabling most things in compiz-config settings helps.