Electron means they're not wasting your heard-earned money. They're hardly spending any of it (on the desktop app, that is).
I’d rather Fastmail ship their web client in Electron than waste engineering resources on reinventing the wheel for a client that’d get it’s own share of endless complaints from people wanting it to be more like Outlook or Thunderbird or Mutt.
But I admit that you got me curious, do you really expect a substantial increase in power consumption due to Fastmail getting a % of their customers to run Electron instead of Thunderbird? I'm really bad at this kind of back-of-the-envelope math but I struggle to imagine that it's substantial.
Like wouldn't one kid in Nebraska playing an AAA game in anger for half an hour, on one computer, once a day, use more energy than all those electron installs combined?