Where does the spell checking function comes from?

Question

1. I am using Firefox. Whenever I type my questions on stackexchange sites, there will be automatic spell checking. Does this function comes from Firefox, or from stackexchange sites, or somewhere else?
2. If it comes form Firefox, why is there no spell checking when I type in some other websites? Just off your head, in what cases, there will be spell checking and in what cases there will be no?
3. American English and British English may spell some words in different ways. For example, if I type "optimization", it will be automatically underscored to let me know the spelling is wrong. How can I make the spell checking tool to recognize American English spelling?

Thanks and regards!

Answer 1

1. As I type this answer on webapps.stackexchange.com, Firefox is providing the spell checking.
2. It depends on the website. I suspect that any site that uses standard textarea tags will allow Firefox to spell-check it. Google Docs, on the other hand, uses its own custom spell checker.
3. Right-click a misspelled word. Select "Languages" and make sure "English / United States" is selected. If you don't see it there, select "Add Dictionaries..." and install the "English (US)" dictionary.

Answer 2

When ever a user type, a spell check program runs behind the scene, and it try's to match with the its local dictionary, if the user entered word is not in its dictionary then the word will be highlighted with the red line, and if you right click that word, you will get an possible combination of its closest matching word. Say for example, if you have typed teh then you will get the possible combination as the.

In short words, a spell check program runs behind the scene that makes us comfortable:) And i guess it depends on the site, which enables this option:)