This also comes from the FAQ specifically about Firefox's spell checker on EpicSki:
Why can't I right click and use Firefox's spell checker?
Because we use a fully WYSIWYG (What you See is What You Get) editor, the right click is needed to be able to pull up image and table properties, cut and paste, etc. However, fear not. There is a work-around to be able to keep using it.
If you hold ctrl (PC) or Command (Mac) when you right click in FF, it will treat the right click as though it's outside of the text editor.
This is an extra step for folks, but the idea was to provide the power of the context sensitive menus in the editor (to align images, add borders, color table cells, etc.) but also enable you to take advantage of good 'ol Firefox. Hope that helps.