[nomel]

Modern browsers don't keep those 200 tabs in active memory. Hasn't been the case for over a decade.

[WhyNotHugo]

But they do keep the active tab of each window in memory. Firefox even continues rendering all active tabs in all windows, even if for windows which are not visible.

Not sure if this 45MB is per browser instance or per tab, but it’s the latter case, 10 windows would save 450MB. >10% on a lower-end device.