BarTap for Firefox keeps background tabs from loading until you use them
Here's what it does. When you click on a link you've chosen to open in a new tab, Firefox normally loads the page content immediately. BarTap keeps an eye on these actions, and intervenes when it sees a new tab appear. It prevents Firefox from loading any content until you actually click over to the tab -- saving resources you might rather allocate to tabs you're actively using.
It even works after a browser restart, and it can make quite a difference in how quickly Firefox launches depending on how many background tabs you have waiting to load.
One more bonus: since they're not loading, those music and video players you have opened on tab 34 won't automatically kick in and blast something embarrassing out your speakers. Win!