Google has just announced that the new Gmail compose box will become the only option within the next few days
A few weeks ago we added a full-screen option to the new compose. Now that that option is available, we’re saying farewell to the old compose and switching everyone to the new compose over the next few days.
The new compose opens drafts as a minimizable window so you can write multiple drafts at once, keep an eye on incoming email, adds support for inline images and much more says Google in a Gmail Google+ post