Mozilla Firefox 4.0 and later versions allow you to move and customize almost everything using Toolbars Customize option but one thing which can't be moved is the newly introduced orange "Firefox" button.
Its big and you can't move it anywhere. It just sits just before the Tabs list.
"Movable Firefox Button" is an awesome add-on which shrinks its size and also allows you to move it. You can move it to Navigation toolbar or Menu bar as per your convenience.
You can download and install it using following link: