Those three operations should make the text rendering correct: Disable hardware acceleration Set gfx.canvas.azure.backends to direct2d1.1,cairo Set gfx.content.azure.backends to direct2d1.1,cairo https://renderingenginetest.netlify.com/ Dark theme can be tested by reversing the foreground and background colors in the HTML. For reference, here is the correct GDI rendering of this website: https://i.imgur.com/dWuZfbD.png https://i.imgur.com/M4oKd7i.png In Firefox ESR 52.4.1 (32-bit), text renders correctly: https://i.imgur.com/InrXmeI.png https://i.imgur.com/kn3bZra.png which is the same as the GDI rendering of this website, except for some line spacing issues. However, in Firefox 70.0b6 (32-bit), the rendering regressed and text no longer renders correctly: https://i.imgur.com/Jn2NxGJ.png https://i.imgur.com/qDxww7y.png Line spacing is correct, but don't let this distract you from how profoundly unsmooth the rendering is in the light theme version. The dark theme version is nearly identical though.