I have a custom button add-on for Firefox that will go to Facebook in the currently viewing tab and would like to add middle click to open Facebook in a new tab but im not sure how. I looked around and found that I need to use the if (event.button == 4) {...} & window.open(url) but im not sure where to put them to get it to work.
Here is the current JS code.

CustomButton = {

1: function () {const url = "http://www.facebook.com"


document
  .getElementById("content")
  .webNavigation
  .loadURI(url, 0, null, null, null)   
  },

}

Thanks for any help and guidance.
Note: If you see anything else wrong with the code, feel free to mention it.