Unlike all the other popular browsers, Firefox does not have a JavaScript debugging console built into the browser. Fortunately there is an addon extension that is available to allow you to add this functionality to the browser. The first step with Firefox is therefore to get the Venkmann JavaScript Debugger and install it into Firefox.
To install the debugger go into the Tools menu and select the Add-ons option. That will open the window or sidebar where you can then select the Install option and go to where you downloaded the extension and select to install it.