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.

Oh ok, great. For me the default scaled works best visually and I don’t use an external monitor so glad I won’t have this issue.

With the handoff working too I’m downloading tonight to give FF a fair shake.