So I came up with a solution on my own. (yay) :) Here's what I did...
- Type about:config in the address-bar
- Enter middle in the "filter" area. You will see a few options
- Right-click on middlemouse.contentLoadURL and click on Toggle. This will now become bold, and it will show "user set"
And thats it! Now I can middle click on a tab to close it. :)
Warning: Do this at your own risk