Rated 4 out of 5 stars

@Jillie Ace

It is for spell checking text boxes/web forms etc. After installing any incorrectly entered text in text boxes and web forms are underlined and if the word is highlight and right clicked a menu shows with suggested corrections.

There is a spellbound extension which improves spellchecking and makes it more like older versions of FF as they used spellbound too.

https://addons.mozilla.org/en-US/firefox/addon/9207

This review is for a previous version of the add-on (1.19).