I always had this problem as I needed to refer to old messages while I compose a new message. One option I had is to right click on the inbox and open it in new tab. Second is to save the new message as draft and look for the old message copy what you required and open back the graft. Now with this new fast Windows feature you have a better way to refer back to old emails while composing a new message.
here are a number of places you can pop up new windows in Gmail.
In chat, there's the little upward arrow in the title bar:
When writing a message, hold the "Shift" key while you click on the Compose Mail, Reply, Reply All or Forward links and you'll get a new window for your new message. (Holding the "Shift" key while typing the keyboard shortcut — in other words typing "C" "R" or "F" — has the same effect.)
When you're reading your mail, hold the "Shift" key while you click on a message to open the conversation in a new window. (Same holds true for the "Shift" key and the "o" or "Enter" shortcuts.)
If you're reading an email and want to save it for later, you can click the "New window" link in the upper-right hand corner of the conversation view:
Keep in mind that the popped out window does not outlive the closing of the original Gmail window, although we're working on a way to make that better. Unfortunately, we weren't able to make this work in Internet Explorer, so to see the speed-up, you'll need to be using Mozilla Firefox, Apple's Safari, or Google Chrome.
Official Gmail Blog: Fast new windows
