hide alt tag in firefox

After trying all the other methods here, I found this method works best which makes the text transparent until the image loads:

.yourClass img {
    color: transparent;
}

The way to prevent alt attribute values from being displayed is to remove the attribute.

The meaning of an alt attribute (not tag) is that it specifies an alternative, a substitute for the image, in situations where the image is not displayed. So if you want to hide it when the image has not yet been loaded, you are asking for behavior that contradicts the very meaning of the attribute.

You can however make the alt text invisible (with the usual CSS Caveats) on Firefox by setting e.g.

img { background: white; color: white; }

in CSS. This implies that the alt texts are invisible also in case the browser never gets the image, or the browser has been configured not to display images.