I got to thinking that since no one else had mentioned this, it probably wasn't a Firefox issue so I disabled all my add-ons and tried it again. As you can see, it works fine.
I'm using Adblock Plus, Flashblock as well as a few others. Later this evening I'll have time to turn them back on one at a time to see exactly which one is interfering with the reply function.
Ok,it's Adblock Plus that got in the way. It's the only one disabled right now and the reply function works fine.