This happens to me at home and at work on laptops… all are Windows NT using Mozilla.
If you click anywhere outside the ‘reply’ or ‘post’ text box, you loose your cursor. You can still type at the end of the line, and can highlight text, but cannot delete/cut/paste or MOVE the cursor to type except at the end of the line.
If you click ‘go advanced’ you get the cursor back–at least until you click outside again (i.e. to add a smiley, or scroll down, or look back up at a post, etc.)
I have deleted all cookies, cleared cache, and all the tricks I know how to do. Is there a Mozilla fix?