The Window and Tab Limiter add-on for Firefox will automatically close old windows and tabs. This is so you don’t end up with too many windows or tabs opened up at the same time. This will save you a lot of CPU resources. You can also password protect the add-on so nobody messes with your settings.
Here is a little more about the add-on from the developer:
After rewriting a business system from scratch as a web application, and switching all users over to Firefox, the one problem I wound up with is that a lot of users would never, ever close any windows. By the end of the day, some users would have 20 or 30 browser windows open and wonder why their computer was running slow. They also got very confused trying to navigate through all those windows. So I wrote this extension that lets you specify the maximum number of windows (or tabs) that can be open at any time (The default value is 10).
This is a really great theme that has a lot of practical uses. Want to learn more? You can pick up Window and Tab Limiter from the Firefox Add-ons website.