Google just made the new pop-up compose the default, and most people don’t like it. At least for now there is an option to switch back to the old compose, although Google does warn that it is temporary. Here is how you do it:

Go to compose a message under the new system. At the bottom right there is a little down-arrow that you need to click to pull up the “More Options” menu.

In that menu, just click “Temporarily switch back to old compose.” You will then get a pop-up confirmation. Accept and your Gmail will refresh with the old compose reinstated.