I'm getting that with Gmail and 2 google sheets open (just as an example workload), my system says Zen uses 899 MB of memory, while Firefox uses 1261 MB. However, the way they split tasks into different processes (or at least the way my system monitor groups them) seems to be different, so I'm not sure how much of that difference is real.

Looking at the browsers task manager, they report about the same amount of memory by the browser itself, and for tab handling FF seems to grab more memory when opened, then decrease over time, whereas Zen seems to have a more consistent memory consumption. Sheets tabs use equivalent memory in both, and Gmail uses about 20% more memory in Zen. Both use an insignificant amount of CPU, of course.

Zen does feel more responsive, but it's not a dealbreaker. Good to know the customizations aren't causing catastrophic resource usage though.

Edit: My only other thing I find wierd is that its kinda hard to close tabs. You have to use the right-click menu -- even using the 'c' keyboard shortcut only selects it, and hitting it again moves to another option!