There is an outstanding bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1429522 that I became aware of due to a Firefox related topic a few months ago[1].

You can improve the performance by setting "gfx.compositor.glcontext.opaque" to true in the about:config settings. This will disable the transparency of the window, but it will reduce the overall resource consumption. The reduction is significant, approximately several extra hours of Firefox use, and your lap will thank you.

[1] https://news.ycombinator.com/item?id=17934764