I'm using that exact combo, Fedora 23, Firefox 45 and Gnome 3. Basically vanilla Fedora 23. I have no issues.

Given how common that font is and how well visited that page is I think it's pretty obvious your system is broken.

Firefox 45.0.1 on Arch Linux running KDE Plasma 5.6 on X here. The font renders fine for me.

Most of the times I've seen fonts break like this, it's because people have outdated local versions of the fonts installed on their system.

Can you check to see if you have the font installed locally, and if uninstalling it (or updating it) changes the behavior?