Mozilla released a 64-bit version of Firefox at the end of 2015. However, like Google, Mozilla hasn't automatically upgraded everyone to the 64-bit version of Firefox. You may still be running the 32-bit version. To check if you're running the 64-bit version of Firefox, open the Settings menu and head to Help > About Firefox (The "Help" button is the question mark icon at the bottom of the menu). You'll see either "(32-bit)" or "(64-bit)" displayed to the right of the version number here.