When Firefox 11 was released it introduced a problem with the way it renders the page edit view on some site configurations, meaning that the page content looked blank.
This affected all sorts of different content management systems. However Firefox 12 now fixes this issue.
If you are using Firefox and find that when you edit a page a section of text is not showing the content as normal, that if you move the expand handle in the bottom right hand corner of the text area even the slightest amount then the content will appear.