The attachement shows roundcube on my 30 inch widescreen monitor. Notice that there is a LOT of space left on the gray bar. My suggestion is simple; detect my browser's screen width (screen.width in JavaScript) and if space permits, display the options which are currently only visible using a dropdown menu (IE Mark and More) as individual buttons. Use a separator bar and title to divide this into sections.
You can see
what this idea looks like in this photoshoped image. (the forum would not allow a 77kb upload)
Keep up the great work!