This is not new, but I just saw it the other day, installed it, and liked it: Tab Effect. It's a plug-in for Firefox that adds a rotating cube effect to your screen when you change from one tab to the other. So, from a distance, people looking over your shoulder might think you're user switching on a Mac or using a much geekier Linux-based OS. And actually, I find Tab Effects adds a bit of situational awareness to my tabbed browsing experience, since the rotation of the cube matches the direction the new tab is from the one you're switching off of. Click on a tab to the right of where you are, your cube rotates right. That's my justification for sticking with this download.
If you want to do more (much more) with tabs in Firefox, check out Brian Cooley's video, You Think You Know Tabs? on CNET TV.