I'm new in JS and need help!

hi there
I'm new in java script and recently work on a project, its drag and drop interactive and the codes will load a random background image and a gif at each drag and drop box. everything works fine on google chrome but the project will not load the backgrounds in Firefox.!
is there something wrong with codes or firefox ?!
  stop: function(event, ui) {
var images =...
Code (markup):
I'm new in JS and need help!

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