This add-on brings the body-tag of the current webpage into the contendEditable-mode. Thus the whole page will be treated as editable content and therefore the whole text will be spellchecked.
The add-on requires the Forefox-Spellcheck-Feature to be enabled. You will also need download the language files for the Firefox-Spellcheck-Feature.
This only applies to verison 0.2 which is available by clicking on "Versions" before it will get reviewed: A "SpellCheck" button will appear in the statusbar. A click on that button will enable SpellcheckEverything and indicate the status by changing its color to green. Turn it off by a second click on that button.