Interesting, as this is the same MBP I have, and same Firefox version. I now have a perf file showing the issue. Can't upload it (it gives an error), but would be happy to send it to anyone who thinks they can help.
I'm on Mac OS 10.11.6 (15G22010). The regression might be due to some interaction between Firefox and changes in the OS, if you're on a more recent version.