I'm always having to look this up on drupal.org, so I thought I'd just put it here once and for all.
(FWIW, you can read about it here: http://drupal.org/node/764100 among other places.)
To enable the firefox spell check when using the fckeditor module, add these lines to the file
FCKConfig.FirefoxSpellChecker = true ;
FCKConfig.BrowserContextMenu = true ;
I always add it to the very bottom of the file so as not to confuse the update process as this is a diff to a file under git control.
For the ckeditor module, it's much the same. Add these lines to the file
config.disableNativeSpellChecker = false;
config.browserContextMenuOnCtrl = true;
With CKEditor, you have to use the control key with the right-mouse-button to bring up FireFox's context menu (with alternate spelling choices) as the CKEditor itself has a right click context menu.
Comments
Spell checker configuration performed by drupalManageLibraries.
The 'drupalManageLibraries.sh' script, part of the Drupal Script Library, will now make the configuration changes necessary to support the built-in spell checker automatically as part of its installation of the external library. (The functionality can be disabled via command line switch.)