Firefox does not use Core Animation, unlike Chrome and Safari, which causes an extra full-window blit on GPU in some cases. It also uses a transparent window, so macOS has to composite the content behind the window whenever Firefox draws. I have a patch in there to improve things, as well as a crate planeshift to help the situation with WebRender, but the patch causes some usability regressions so it has not landed. In the meantime you can set gfx.compositor.glcontext.opaque to true in about:config to get an opaque window at the cost of rounded window corners and vibrancy.
Firefox does not use Core Animation, unlike Chrome and Safari, which causes an extra full-window blit on GPU in some cases. It also uses a transparent window, so macOS has to composite the content behind the window whenever Firefox draws. I have a patch in there to improve things, as well as a crate planeshift to help the situation with WebRender, but the patch causes some usability regressions so it has not landed. In the meantime you can set gfx.compositor.glcontext.opaque to true in about:config to get an opaque window at the cost of rounded window corners and vibrancy.