Steps to follow
1) Open a new tab in Firefox and in the URL bar, and enter about:blank to display a blank tab so that you can see what you are doing.
2) From the Firefox menu select Tools -> Web Developer -> Web Console or use key combination of Ctrl+Shift+k
3)A new Web Console window will open. From the Web Console window click the JS button on the toolbar.
4)From the Web Console window click on the Scratchpad icon in the toolbar. The Scratchpad icon looks like a notepad.