The menu listing paragraph styles (Heading 1, Heading 2...) is now automatically charted according to the actual styles in the online editor.
If this menu was already charted on your previous version, you need to remove the CSS rules that are no longer required (the ".mce-container.mce-menu XXXXX" rules). For example:
.htmleditor-h1, .mce-container.mce-menu .htmleditor-h1 { ... }
.htmleditor-h1, .mce-container.mce-menu .htmleditor-h1 { ... }
.htmleditor-h1, .mce-container.mce-menu .htmleditor-h1 { ... }
becomes
.htmleditor-h1 { ... }
.htmleditor-h1 { ... }
.htmleditor-h1 { ... }
More details here Integration manual (in the section Customize paragraph styles in the rich text editor)
Check the menu's appearance in Front Edition: in most cases, the automatic appearance will give good results and be a real plus over a static menu; but it is possible, in some cases, for the menu's appearance to be distorted, in which case you'll need to go through customization to correct these bugs.
You can also customize it to obtain a preview closer to reality.
More details here Integration manual (in the section Customize paragraph styles in the rich text editor)