For me it is only slow if I go outside the default scaled resolution. On my 2015 13" mbp I use the default scaled resolution to get around the performance issue

For me it is only slow if I go outside the default scaled resolution. On my 2015 13" mbp I use the default scaled resolution to get around the performance issues. Another work around if you want to use a non-default scaled resolution with firefox is to use firefox in non retina mode. I prefer not to use this as it makes the experience blurry. I do wish they would fix this issue though.