Mac El Capitan, FirefoxDeveloperEdition (which iirc is basically equivalent to Firefox Beta), not many addons (Tab Groups, uBlock Origin, No More 404s). I can screenshot my Activity Monitor if that'd help?
Image PID Hard Faults/sec Commit (KB) Working Set (KB) Shareable (KB) Private (KB)
firefox.exe 10604 0 1,529,308 1,491,692 62,348 1,429,344
This Firefox instance is running since: 8:05:29 AM 05/12/2016
Firefox 50 (with a pending update).
Also, I see the process as a 32-bits image but I was sure it was 64-bits, will need to check on this.
And I have only: Facebook Ads Block, Ghostery and uBlock Origin.
So being Windows 7 and probably an 32bits Firefox install may cause all this difference?