Firefox, I love you, but you and my 15" rMBP (with 4k Dell external monitor) simply don't get along: https://i.imgur.com/rByz7zS.png I don't what causes this, but in the last 2 years it's gotten worse and with Quantum the CPU usage is even higher. Chrome and Safari don't have this issue.
Although performance is greatly improved (and, seriously, the developers should be proud of the progress), there's still a lot of work to be done. There's significant lag in even non-network UI (e.g., highlighting the row under the cursor in about:config lags behind the mouse cursor by almost a second).
As much respect as I have for the developers for this progress, I still can't really use software that makes me feel like I'm intoxicated.