Firefox Giving DOM Security Error 1000

Firefox is throwing a DOM security error. I've used toDataURL() before, and it's never done this. I need to get this little app fixed, so that I can use it on my TabletPC for taking notes in class. The line of code that is throwing this error is:
var Note = document.getElementById("SketchPage").toDataURL();

Here's the full error from the Error Console:
Error: uncaught exception: [Exception... "Security error" code: "1000" nsresult: "0x805303e8 (NS_ERROR_DOM_SECURITY_ERR)" location: "file:///C:/SketchBook-Dev/SketchBook.js Line: 236"]

Here is the JS file:
var PenSize = "3";
var PenShape = "Circle";
var PenColor = "Black";
var LoadFile = "";
var UIstatus = "visible";
var CurrentNote = 0;
var BGcolor = "#C7C1A3";
var DataPath = "Data\";
var ImageExtension = ".img";
var FileList = [];
var SystemPath;
var UIstatus = "visible";
var Server = "localhost";
var NxtNote = new Image();
document.onkeyup = ToggleUI;
function Init() { .....