Taking this as an opportuinty to pitch an extension I developed to do that and more: https://addons.mozilla.org/en-US/firefox/addon/web-defuser/

The main difference between this and current element blockers is that Web Defuser allows you to block annoying behaviors (by modifying requests/responses in flight) in addition to elements.

At the moment it's a bit lacking in the UI department, I'd appreciate early adopter feedback (you can contact me at gmail with my username).