Ok, so it's not breaking into top-secret government computers, or waging DDoS war against eBay, but it has its uses. It's just a few cheap tricks to make web browsing a little easier and more fun, and that's what hacking is really all about**.
Mozilla supports a custom style sheet that can be applied to every page you view. With a little crafty design, you can kill (some) banner ads, show which images are links, and other fun tricks.
img[width="468][height="60"] {opacity: 0.1 !important;} This line will make any 468 x 60 image fade into the background at 10% visibility. (as of build 2001041808, this doesn't work. I think it's a bug in the new libpr0n).
A:link IMG, A:visited IMG {border: 2px solid blue !important;} This puts a blue border around images that are links. This might look ugly on some sites that use several images together (like the everything2 logo at the top of this page) I usually don't use this one.
A:hover IMG {border: 2px solid red !important;} This one is my favorite, when the mouse pointer moves over a link image, a red border pops up around that image. No more pixel-hunting.
These hacks and others even more bizarre and useless are in "The CSS Anarchist's Cookbook" on the O'Reilly network.
** note: please please please don't turn this node into a debate about the meaning of "hacker"/"hacking" vs "cracker"/"cracking". There's already a node about that, fool! ;-)