Hi. I have just discovered that there are internet pages that use your processor 100% by javascript to generate money without your permission for the web.
Example of javascript mining web: http://www.elitegol.me
It is not dangerous, simply while visiting that website the processor is set to 100% and consumes more electricity. The latest versions of Firefox and Chrome are already protected, but the latest versions for Windows XP that are Firefox 52.6.0esr and Chrome 49.0.2623.112m are not.
Web to test Windows XP browsers: https://cryptojackingtest.com
To block the mining, extensions can be placed. One that I have tried and it works well in FF and Chrome for XP is called: Mining blocker