JavaScript is a programming language, a dialect of ECMAScript. Unfortunately,
someone a long time ago decided that it would be a good idea to design a Web
browser to let websites send programs written in JavaScript, and then
automatically execute them, silently. Flash Player does the same with
ActionScript, another ECMAScript dialect. The thing is, this behavior is
incredibly stupid from a security perspective, and these programs also tend
to be proprietary.
I do not recommend LibreJS. As you noticed, LibreJS does slow things down
quite a bit, and I don't think it solves the problem. I've explained why in
detail here:
https://onpon4.github.io/other/kill-js/
Instead of LibreJS, I suggest one of the following:
- NoScript:
https://addons.mozilla.org/en-US/firefox/addon/noscript/?src=search (make
sure to clear out the initial whitelist if you choose this option, since it's
quite large)
- QuickJS: https://addons.mozilla.org/en-US/firefox/addon/quickjs/?src=search
