alt Hacker News>Being able to change stylesheets
https://addons.mozilla.org/en-US/firefox/addon/styl-us/
>disable or enhance various JavaScript scripts
https://addons.mozilla.org/en-US/firefox/addon/ublock-origin...
https://addons.mozilla.org/en-US/firefox/addon/tampermonkey/...
Yeah you can't directly alter scripts being ran (as far as I know?) but you can usually override/extend behavior and can definitely add your own
>add notes and annotations
https://cwmonkey.github.io/greasemonkey/make-note/
(I haven't actually used this one, just first result)
I'm aware of plugins. But these used to be builtin features. Developers needed to work with them, rather than making it harder and harder to use them to make the users life easiee.