Originally Posted by u-Bob

According to the standards, a browser should only show the contents of the alt attribute when the browser can't download/show the image or when the browser is configured to not show images. Inferior browsers like Internet Explorer for example don't follow the standards and do show the contents of the alt attribute when you hoover over an image. Firefox does follow the standards and does not show the contents of the alt attribute when you hoover over an image.
(Firefox does it the right way)

If you want to show some text when you hoover over an image, you should use the correct attribute; the title attribute.