Sometimes when I type in the body WYSIWYG on Firefox I can only type one character at a time. I've tried this on the latest stable versions of Firefox on OS X and Windows 7 and Windows 10. This issue I believe has been around for some time now. Steps to recreate would be to write something, then select all text and cut it. When you try to type you will only be able to type one character before focus is lost. I just tested this now and had the same results.