Ladies and gentlemen, let me introduce to you a series of Firefox add-ons called "Spelling Checkers". First, what is an add-on: it's a small software embedded on the Firefox browser and enables it give the user additional experience. For example, a video downloading add-on lets the user download videos directly from the browser.
So what are the spelling checker add-ons: they enable the user select a text on a web page and immediately get a spelling check performed on this selected text. The misspelled words are marked in bold, so the user can know where are the spelling mistakes.
Why use these spelling checkers instead of spelling checkers implemented in Windows software or websites? In my opinion, it's just much more convenient: the user doesn't have to switch web pages let alone open a software. The user just select a text and clicks on an icon located on the browser.
What are the drawbacks of these spelling checkers: they are based on a vocabulary database, which may cause "false positives" i.e words being diagnosed as spelling mistakes without a reason. Why? plural forms of words (for instance) can mislead the spelling checkers, which simply compare the selected words against the vocabulary database.
What languages are covered by these series of spelling checkers? obviously, the most spoken western languages are covered: English, Spanish, Portuguese, German and Italian. Future spelling checkers may deal with languages like: Russian, Arabic, Japanese and Korean.
The spelling checkers are technically supported by email. As I tried this support personally, the user can get a response within a day and the support is quite comprehensive.