Steps to reproduce:
1. On Firefox (version 36.0), access PlatformUI;
2. Go to "Admin Panel -> Content Types -> Content";
3. Click on "Create a Content Type" at the bottom;
4. Create a content type called Rich Text with Name and a Rich Text field;
5. On "Content -> Content Structure" create a content of the type Rich Text;
6. On the rich text field, click on the "Add Content" icon at the bottom and verify that the sub-icon "h1 Header" doesn't show up above it.
If it shows up, try again (sometimes it shows once but not again thereafter);