still had the same problem.  Here's what I did

1) Go to Table > Properties



2) Go to Style Builder > Border.  Selected Solid border.  Click OK
 

3) After clicking OK, I see the border in Normal mode.   actually now, when I switch to HTML mode I see the html code for the border.  but if I switch back to Normal mode, the border is gone.  and now, if I go back to html mode the html code for the border is gone.

so it looks like toggling back and forth a couple of times between normal and html (or even preview) mode causes the border to disappear in FireFox.