On many sites, the PHP giveth while the JavaScript taketh away. The page content is perfectly fine -- until a bunch of scripts fire and festoon the site with confusing, time-consuming pop-ups.
The NJS extension is a solution that works well for a surprising number of sites. It leaves JS on by default, so unexpected breakage doesn't happen on sites where the user expects JS to work, but allows the user to turn JS on and off and have the choice persist.
https://addons.mozilla.org/en-US/firefox/addon/njs/
("Reader mode" is similar in some ways, but there are sites where "reader mode" is not as usable as the normal layout minus JS)
A lot more Firefox users could benefit from NJS if Mozilla could add it to the Recommended Extensions program and/or provide security reviews, so that the "This add-on is not actively monitored for security by Mozilla." warning could be removed.
Although NJS is currently not a recommended extension, it's relatively simple and appears to have few updates.