My apologies to the Bugzilla team for wasting their time holding my hand on this one. Would have honestly never noticed the little “HTML5” info icon to the left of the URL bar though without their help.

  • Different graphics stacks (graphics hardware, their drivers (of which there are different ones for all major OSes), the display server (on Linux)) draw things slightly differently. As I remember this especially applies to text rendering using different fonts, but slightly tilted lines, bent lines, and color blending is probably also part of it, and more.