How do I...Enable Firefox's Spell Checker?

Caveat: This is an early solution for people who cannot wait. This will only work for FCKeditor, and will not be forward compatible (the next time you update, you will have to manually revert this change). We will be providing a maintainable long-term solution for this problem in Killen Woods. 

Open up your FCKconfig file: /editor/fckeditor/editor/fckconfig.js and add these two lines at the bottom:

FCKConfig.FirefoxSpellChecker    = true;
FCKConfig.BrowserContextMenuOnCtrl = true;

Be sure you clear your server-side UI cache (delete all files in /skins/common/cache/ with a cache-* prefix) and give your browser a hard refresh.

This will start showing the browser squiggly lines; to activate the context menu for Firefox, Ctrl + right click.