Neither of those bugs have to do with 100% CPU on rMBP. Their about using an inefficient presentation path which increases GPU power usage. You can mitigate this today by setting gfx.compositor.glcontext.opaque to true. Further, Firefox Beta has additional texture upload performance improvements that will reduce cpu usage.
If you're actually experiencing 100% cpu usage, post a profile using https://perf-html.io/ and I can try tell you why.
If you're actually experiencing 100% cpu usage, post a profile using https://perf-html.io/ and I can try tell you why.