Sometime around April, Google changed the mail compose window into something that has been hated by people ever since. Initially, Google offered users an option where they could revert back to the old compose window temporarily. But now changes are permanent. All is not lost though, especially if you are using Internet Explorer 8 or Chrome.
Apparently, the old compose window is still available to Internet Explorer 8 users because the new interface is not supported by the old browser. By taking advantage of this little piece of information, one can get the old compose window back. All you need to do is change the user agent of whatever browser you are using, so as to trick Gmail’s servers into believing you are using Internet Explorer 8.
Retro Compose for Gmail and Classic Gmail Compose are two extensions for Chrome that does that. While Classic Gmail Compose permanently changes the compose window to the classic style, Retro Compose for Gmail offers a toggle button that allows you to switch between the old and the new compose window.
Keep in mind that this is only a temporary arrangement. Both extensions will continue to work as long as Google serves the old compose window to Internet Explorer 8. At some point, Google will stop supporting Internet Explorer 8 altogether and these extensions will stop working.