Actually the latest version is Firefox 66.0.3, which just came out yesterday. But yes, you should be using that version. What seems slow? One thing you can try is seeing if your computer will update to a newer version of MacOS. You currently have 10.11, You can see if you can update to a newer version. This may bring performance improvements. Another thing you can do is install an adblocker, such as https://addons.mozilla.org/firefox/addon/ublock-origin/ which will likely bring performance improvements