Problem is with the contentEditable="true" value (SP uses this extensively). When contentEditable="true" is used, CSS select (-moz-user-select:) becomes unreliable.
Attempting to force the select text to be editable via CSS has no effect:
-moz-user-select: text !important;
A workaround I figured out was to place the cursor at the end of the content, hit backspace, then the content becomes selectable/editable.
I would recommend logging this as a know issue.