I am guessing you click on Write and the composition window is not full frame. Thunderbird opens just like you closed it so if you minimized the composition window last time and closed it that is how it is going to open. Click the maximize button. Is it better now? If you have lost your toolbars press the alt key to show the menu bar if it is not already showing. From the menu bar select View-Toolbars and turn them back on. Send is on the Composition toolbar.