I am using the firefox developer edition since a year and a half. I have noticed that
everytime there is a firefox update is available, the memory and the CPU usage for the browser is fly rocketed. The tabs gets unresponsive and hangs up the machine. I am on macOs Sierra- 10.12.3 - slowness or unresponsiveness is regardless of the site I am visiting.
When running into problem like this, I usually record a performance profile with Nightly using the add-on and submission site at https://perf-html.io/ Then I’d submit a bug at bugzilla.mozilla.org with a link to the uploaded performance profile so that developers can look into this.