There is no possibility to add headers to the table inserted in Rich Text Editor using Firefox (Chrome and Safari are not affected).

Steps to reproduce:
1. Create a new Content having at least one RichText Field Type.
2. Insert a new table inside.
3. Try to add headers to the table (click on any option from the toolbar e.g. "First row").

Result:
Headers are not added, there is no effect (no <thead> section is added).