I really like
NoScript: stops any and all JavaScript from running, which cuts out the main browser annoyances. In the current version you can choose to allow scripts on the site you browsed to (whatever it was), just not 3rd-party add-on scripts - which means that "Web 2.0" type sites usually work fine. It also has an "untrusted" list, to permanently blacklist known rubbish such as trackers or ad scripts.
Another useful one is
PDF Download: why would I ever want a PDF to be displayed inside Firefox, where the navigation keys don't work?
