Firefox has had this feature for over a decade, and it has never worked reliably. It's slightly better now, but still one time in five or ten, it starts up in a blank session or with an error page prompting you to press a button to restore state.

I lived through the days where force closing Firefox was the only way to preserve sessions. Things honestly aren't that much better. I'd take a zero percent chance of success over 80. At least then failure is known and predictable instead of just reliable enough to get you to trust it.