I tested it as well. I started with java 1.5.x.
In Opera, text fields would not take focus (i.e. could not type anything anywhere). Couldn't even enter a chatroom.
In SeaMonkey, text fields took focus normally. I entered a chat room and tested private chat and, as nightflier said, hitting 'enter' did nothing. Had to kill SM to exit.
Did not test Firefox.
I updated Opera and Java (1.9.24 and 1.6.0_03, respectively) and retried.
In Opera, text fields took focus normally. I entered a chatroom and tested private chat and but 'enter' would not work. Had to kill O to exit.
In SeaMonkey, text fields took focus normally. I entered a chatroom and tested private chat and this time 'enter' worked correctly sending the message. However, I still had to kill SM to exit.
In Firefox, exact same as SeaMonkey but response time was much better/quicker :shrug:
SeaMonkey and Firefox are 1.1.4 and 126.96.36.199, respectively
The chat rooms are, at least, usable but it's a paint that you cannot close a private chat window. I'm guessing that it falls on poor java coding - but it just a GUESS so no flames please