This add-ons is basically to check the Spelling mistakes all over the web page. It uses Firefox's inbuilt dictionary to check the spellings. There are many sites available which checks the spelling mistake on the website but for that you must have your site live. but what to do when site is under construction and you are testing it.To copy whole page text and past into some utility that checks the spelling mistakes is tedious work to do. This add-on will help you in this case.
To check spelling in other languages you will have to download the dictionaries, you can down it from here.