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:
controls.css:
textarea.ms-long,
#ctl00_m_g_38973098_a9bc_4017_aac1_d93a217166d1_ctl00_ctl02_ctl00_ctl02_ctl00_ctl00_ctl02_ctl00_ctl00_ctl04_ctl00_ctl00_TextField
{
-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.