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.