Is there any way I can use HTML5 and JavaScript to modify a file on my system? I\'m OK with running Chrome or Firefox with specific options or opening permissions i
Yes, you can do this, this link details some of the available functionality:
http://www.html5rocks.com/en/tutorials/file/filesystem/
Just remember, it's not supported in the major browsers yet, and it's likely when it is many of your visitors will not upgrade for quite some time.