Supports the latest Firefox + Greasemonkey and Chrome + Tampermonkey.
You can tweak which elements the script hides by deleting lines in
globalElements or editing sidebar block headers in
sidebarBlocksGood (all other blocks are hidden; applies only to movie/person pages). It also removes the IMDbPro button, you can remove the relevant line in the script's source code if you need it.
Example: before / after (clickable)