Why does the firefox debugger jumps to a random location?

Question

I have this piece of code in file-A.js

SugarBee.prototype.doubleClicked = function(canvas){
    debugger;
    this.createPropertyEditor();
}

Firefox debugger stops at the first line of my function, which is what I want.

So I hit the "step in" button twice, and I expect firefox's debugger to take me to the createPropertyEditor function definition, that what should happen, right ? but instead, the debugger jumps to a random location in file-B.js.

AjxDispatcher.require=function(n,a,h,s,r) {
    if(!n) {
        return
    }
    if(typeof(n)=="string") {
        n=[n]
    }
    var l=[];
    for(var o=0; o<n.length; o++){
        var t=n[o];
        if(!AjxDispatcher._getPackageData(t)._loaded) {
            l.push(t)
        }
    }

>>>> if(l.length==0) {
        return AjxDispatcher._postLoadCallback(n,false,h,s)
    } else {
        var e=AjxDispatcher._preLoad;
        if(r&&AjxDispatcher._loadFunctionsEnabled&&e.length) {
            AjxPackage.__log("pre-load function");
            AjxDispatcher._timedAction=new 
                AjxCallback(null,AjxDispatcher._continueRequire,[l,a,h,s]);
            for(var o=0; o<e.length; o++){
                e[o].run()
            }
            window.setTimeout("AjxDispatcher._timedAction.run()",0)
        } else {
            return AjxDispatcher._continueRequire(l,a,h,s)
        }
    }
};

The debugger jumps to the line marked with ">>>>"

Why is this happening ?