[version 2026-09-22]

Seems like people want to remove scrollbars?
Seems like fuck those people, what do you care of my scrollbars?

Put the following in your .mozilla/firefox/XXX.default/chrome/userContent.css
------------------
* {
  /* scrollbar visible */
  scrollbar-width: auto !important;

  /* format: scrollbar-color [front color] [back color] */
  scrollbar-color: #555 #ccc !important;
}
------------------

And have this in about:config (adjust to fit your choices)
(some may be not needed, just experiment)

layout.css.scrollbar-width-thin.disabled                true
layout.testing.overlay-scrollbars.always-visible        true
widget.gtk.overlay-scrollbars.enabled                   false
widget.non-native-theme.scrollbar.size.override         50
widget.non-native-theme.scrollbar.style                 1
widget.non-native-theme.win.scrollbar.use-system-size   false

(hopefully I don't forget one, I should have noted down what I changed
when I changed, not several hours later... what a loser...)

Ah, side note, you may want to disable "smoothscroll", that thing is very
annoying, look for "smoothscroll" is about:config and set all to false.

And also to use "backspace" to go back in history, still in about:config set
browser.backspace_action to 0.

Disclaimer: I'm using fvwm and X11. All this may fail to work in your setup.

There may be other ways to do the same, if you know something, I'm happy
to learn! Get in touch.