Rated 5 out of 5 stars

It's great for doing exactly what it describes, quickly figuring out if a word exists on a page, and provides a shortcut for typing the long, foreign, or hard to spell words. It's also configurable (check out extensions.prospector in about:config), so you can change the min word length of suggestions, the max number of suggestions, and the pattern used to split/discover words on the page.

This review is for a previous version of the add-on (3).