Your namespace should be declared as the following rather than with quotes:
namespace folder\MenuComposer;
Composer dump-autoload then generates the following in your "/vendor/composer/autoload_classmap":
'folder\\MenuComposer\\MenuComposer' => $baseDir . '/app/libraries/folder/MenuComposer.php'
Which would indicate the class can be reached at:
folder/MenuComposer/MenuComposer
Hope this helps!