I was gonna try to make a Tamper Data add-on but I dunno how I'd make it work for Flash and AJAX and stuff, and that's what's really important since it's too easy to edit the normal HTML stuff, therefore I made an easier version of it that just converts all hidden fields into text boxes with the colors inverted (black background, white text).
Code:
javascript:(function(){var%20is=document.getElementsByTagName("input");for(i=0;i<is.length;i++){if(typeof(is[i].attributes["type"])!="undefined"&&is[i].attributes["type"].value.toLowerCase()=="hidden"){is[i].setAttribute("type","text");is[i].setAttribute("style","background-color:#000000;color:#FFFFFF");}}})(); Enjoy.