Firefox: It's no secret that Firefox takes up a lot of your computer's resources, and pages that load for a long time can end up freezing or crashing the browser. KillSpinners stops that from happening.
The extension lets you select how long any web page can be loading for, after which it will kill the process and save your computer the burden. By default, it's set to 30 seconds, but you can change that to find the sweet spot.
Of course, you can set exceptions for a few websites that you visit often. And when KillSpinners does stop a website, it will give you a small notification bar informing you about what it did.
KillSpinners is also pretty light -- you can easily check which extensions are eating up your memory. It's a nifty tool to have in your browser, and I wish there was a similar extension for Chrome. If you know of one, drop a line in the comments.