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.