This is an extension to popup alternate texts given as ALT attributes (they are not descriptions, but authors often use the attribute only for popups), like NC4, IE, and other browsers.
As an additional feature, this includes "multi-row tooltip". If the text is too long, it will be shown in multiline automatically.
If you use Mozilla Suite or Netscape 7, see How to install and update? (contains important notes) and How to uninstall by hand?
user_pref("browser.chrome.tooltips.attrlist.enabled", true); user_pref("browser.chrome.tooltips.attrlist", "alt|src|data|title|href|cite|action|onclick|onmouseover|onsubmit");
By the secret settings above (by the "user.js" or the "about:config" feature), the browser popups all of attributes in the list. If the pointed element doesn't have the attribute, the browser finds a parental node which has the attribute.
And, the following setting shows attributes for all of parental nodes:
These features possibly prevents spoofings and phishings.