In Firefox only, does not happens with Chrome :
If you use the "paragraph style" combobox on some richtext, for example if you set a text to "Title 1", the richtext loses the focus. After that, everytime you try to focus the richtext, the focus is stolen by the ribbon within 1 second.
If you close the tab and reopens it, the focus is still lost every time.
You can keep the focus in the "HTML code mode", but switching back to the standard mode will make you lose focus again.

This bug prevents you from being able to edit the RichText until you refresh your browser's page.