At the same time the security vulnerability was issued, Mozilla also released Firefox 67.0.3 (and ESR 60.7.1) to fix the issue.
Updating Firefox in Fedora
Firefox 67.0.3 (with the security fixes) has already been pushed to the stable Fedora repositories. The security fix will be applied to your system with your next update. You can also update the firefox package only by running the following command:
$ sudo dnf update --refresh firefox
This command requires you to have sudo setup. Note that not every Fedora mirrors syncs at the same rate. Community sites graciously donate space and bandwidth these mirrors to carry Fedora content. You may need to try again later if your selected mirror is still awaiting the latest update.