Estimating Chrome's memory footprint using Task Manager can easily give you incorrect results, because some of the memory is shared between tab/plugin processes. You can get correct data by navigating to this URI: (SU won't let me make it a real link)
Back to your issue. If you have 70 tabs open, it will surely consume A LOT of memory. If it's really 8 GB in total, then it's about 110 MB per tab - pretty good result, I have 4 tabs open right now and my memory consumption is 215 MB per tab.
That's not strange that Chrome consumes so much memory and Firefox had crashed for you, no browser is designed to handle 70 tabs at once. Your brain isn't too, and I doubt you're using all of them at once - my experience tells me that 20 tabs is a lot and about half of them is just hanging there, because something popped up and interrupted your previous task.
You can do yourself (and your computer!) a favor by changing your browsing habits. Get some tab organizing extension, like Pocket/Pickpocket or some tab saver. Less tabs are easier to handle and working with them is more efficient, both for you and for your browser.