Chrome and Internet Explorer should come with the Dictionaries built in. An extra step is required with firefox to install the appropriate dictionary.
Firefox - Install Go to the Add-ons Page (By clicking on the menu button and selecting Add-Ons). Click on the Get Add-Ons tab on the left hand side In the search box in the top right of the Get Add-ons page, type in the dictionary you would like (e.g. "British Dictionary") If you have installed the dictionary previously it will be listed under the Dictionaries tab on the right, otherwise. click on the Install button for your Dictionary of choice.
Firefox/Chrome - Usage While in the editor, all typos will be underlined with a red line similar to using Word. To correct the spelling move the mouse or cursor to the word and press CTRL - Right Click. This will give you a variety of spelling options, including the ability to add the word to the dictionary.
Internet Explorer - Usage While in the editor, all typos will be underlined with a red line similar to using Word. To correct the spelling move the mouse or cursor to the word and Right Click. This will give alternative spellings and the ability to add the word to the dictionary.