The developers of CKEditor have reported that Firefox 39 introduced a regression regarding the text selection and editing on nested content editable elements. Editing on such elements is currently difficult because the caret won’t appear where clicked, the caret doesn’t move with keyboard keys, and non-editable elements are also selected when clicked. Mozilla developers are aware of the issue and working on the solution.
Update: According to a comment in the bug, MediaWiki VisualEditor’s table editing functionality is also affected. This bug has already been fixed with Firefox 40 Beta and Firefox 41 Developer Edition.