This sequence of commands will download the necessary files and execute them to generate the Firefox appimage, then you can access the out folder and click on the generated appimage.
mkdir buildFirefox;cd buildFirefox;wget https://raw.githubusercontent.com/AppImage/AppImages/master/pkg2appimage; mv pkg2appimage pkg2appimage.sh;chmod +x pkg2appimage.sh;https://raw.githubusercontent.com/cmatomic/AppImage-009/master/Firefox.yml;./pkg2appimage.sh Firefox.yml