I've notice this of late too. I've managed to still quote someone by using the "Switch Editor Mode" feature in the top right corner of the message box. For some reason, quoting someone in the one mode works, whilst in the other it does not. I'm not 100% sure which mode is which, but when I have a cream background and typing, I cannot quote; when I have a white background and typing, I can. I'm using the dreaded Internet Explorer 11.