If you work with many tabs in Firefox, you may have some websites that leech away the computer’s processing power even when they are running in the background. Some websites for instance update contents automatically even if they are running in a background tab, others may play Flash contents like advertisement.
The Firefox add-on Suspend background tabs tackles the issue by suspending actions in background tabs. The author of the extension notes that it currently won’t take action on sites with Flash or media contents, but that a future version may introduce the – in my opinion much needed – feature.
install flashblock with it (or uninstall flash) and it works great... this is a definite cpu/battery saver and makes old machines able to handle more tabs....
direct link to it here; https://addons.mozilla.org/en-US/firefox/addon/suspend-background-tabs/