For the chat rooms you need Java. Firefox uses the Sun Microsystems Java engine (which is the original), so if you don't have it installed, chat won't work for you on Firefox. Anyway, FF itself should ask you to download and install Java if you don't have it when you try to open it.

IE installs Micro$oft's Java Virtual Machine. If chat is not working for you, maybe something corrupted your IE installation or you chose not to install MS JVM. Anyway, you can download Sun's Java Engine (I strongly recommend it, even if you have MS JVM) and it should work on IE as well.