firefox has some problem in rendering a “pre” tag in a div tag with overflow attribute set to auto, so to fix it you can just remove those attribute of the “pre” tag in style sheet, this generally happen in different theme you use.
to solve this go to your theme folder(wp-content\themes\xxxxx where xxxxx is the name of the currently installed theme) and find “Style.css” open it with a text editor and search for “pre{” without quote.
you’ll find a line like this:
pre{margin: 20px 0;padding:25px 15px;overflow:auto;background-color:#eeeeee;border:1px dashed #FFCCCC;font-size:11px;font-family:"Courier New",Courier,monospace;font-weight:normal; overflow: auto; width: 305px;}
now in this line remove “background-color” and “border” attributes including their values. so in my case i should remove this:
background-color:#eeeeee;border:1px dashed #FFCCCC;
save it and upload the file. browse your website and refresh your browser cache by pressing CTRL+F5, just to make sure it replace the old style.css file from browser cache.
hope that helps ^_^