It's because your document doesn't have an element with an id of "replybody"
There's only an element with a name of "replybody"
Why it works in IE is that IE automatically creates global variables for elements with names and ids, and some other criteria as well, which does cause problems in its own right.
Solution #1: Use getElementsByName
var currentText = document.getElementsByName("replybody");
Solution #2: Put an id attribute on the textarea
<textarea name="replybody" id="replybody" rows="10" cols="80"></textarea>
Personally I prefer the first solution myself, because it's not adding extra unneeded cruft to the html code.