You can get Charla
here, give the application a try.
Charla is safe, my "not a happy one" interpretation means it's not the 100% solution, but so far is the better. It's a Chat browser, only to visit Yahoo Chat Rooms.
The Firefox problem, like the Safari one, it's a Java error. There used to be a plugin to fix that, but it's included in the last Firefox version now.
And about your last question. There's a lot of chat rooms around, not Yahoo with its old Windows codecs, but you've got to make a search in the web to find the better place for you.
Quadra 605, G4 and IMac 500 Mac OS X (10.4.5)