Hazel doesn't close tabs because it's 'like a trip down memory lane.' Mozilla says it has some profile and tab group features coming later this year that might help her organize those memories.
There's a setting to always have the tabs reopened when restarting the browser.
It also reopens them in an unloaded state, until you actually click on that tab, so that's how Firefox doesn't immediately explode when reopening 7000 tabs.