When composing a message or editing a web page, you may not want to add alt text for an inserted image. No Alt Text is a SeaMonkey and Thunderbird extension that changes the default for new images to add an empty alt text attribute. This corresponds to the "Don't insert alternate text" radiobutton selection.
Additionally the image properties dialog has a bug whereby it fails to realise that you have previously specified no alt text for an existing image. This means that it will nag you to set the alt text of an image, when you really meant to change one of its other properties. This extension works around the problem because it prevents the dialog from defaulting to alt text.
Thunderbird 6 has a workaround for this bug in that the alt text will default to the same state as the last image that you edited. Future releases of Thunderbird and SeaMonkey will contain a fix for this bug.