Visual studio can debug Javascript. You will have to write debugger; where you want to put the debugger. This will open the runtime file in VS. then you can use the IDE to put the breakpoints anywhere. Highlight the variable and see the values in quick watch and everything. else Firebug. Firebug has some learning curve if you are new. you will get used to VS more quickly.
function Test()
{
var a = 10;
var b=20;
debugger;
alert(a+b);
}
Also remember JavaScript debugging should not be disabled in IE (if doing it in IE). Go to advanced options in IE tools menu and uncheck the checkboxes for Javascript debugging disabled.