[masraum]

Noscript and adblock plus.

Also
IE Tab 2 (or IE Tab +, can't remember for sure, both are similar)
Image Zoom (allows you to resize, up or down, images on a page including rotating)
Mobile Barcoder (turn text or links into a QR code for easy scanning into a smart phone)
Tab Kit or Tree Style Tab (allows me to move the tabs so they run down the side of the browser instead of across the top. This is a big one since most monitors are widescreen/landscape, but most web pages are tall/portrait.
At work, we use a proxy, but we have some other proxies that we have to use for accessing other devices in the network, so I use Foxy Proxy for that, but that's a somewhat unusual and specialized thing.