You can save power when you are not working by putting your system under stand by. In standby mode the CPU fans, monitors etc will not work, thus saving power. Almost all operating systems have the standby option.
For enabling and disabling JavaScript you need an add-on for SeaMonkey called quick java . You can download this free add-on from mozdev.org or just Google for it. After installation you will be able to see two buttons near the progress bar, for enabling and disabling JavaScript and java. Just click on the buttons to disable and enable JavaScript or java. There will be separate buttons for java and JavaScript.