Can't download Disable Private Browsing Add-On ~ Not Compatible w/ Firefox 20.0

I am trying to download/install the Mozilla "Disable Private Browsing" add-on/extension. (See this link to see the add-on I'm referring to: https://addons.mozilla.org/en-us/firefox/addon/disable-private-browsing/) However, whenever I try to download it/install it, it says "Disable Private Browsing could not be installed because it is not compatible with Firefox 20.0" How can I fix this?? Is there another Disable Private Browsing add-on that IS compatible with Firefox 20.0?? I really need this add-on feature asap. It is very important that Private Browsing is disabled as a quickly as possible on my laptop. Note: I have a MacBook Pro Laptop - OS X. Please help. Thank you