The #WordPress #Gutenberg block editor has some annoying #Firefox scroll bar issues for years, including vertical (urgh) scrollbars.
This CSS snippet mitigates it for me. I wished I’ve fixed it before.
/*** Firefox vs Gutenberg sidebar scrollbar fix ***/
.interface-interface-skeleton__content,
.interface-interface-skeleton__sidebar,
.components-popover__content
{
scrollbar-gutter: stable;
scrollbar-width: thin;
}