So per-tab content processes (which btw is the only way e10s can result in security and stability benefits) is not a realistic goal for them.
People always say that as if crashes would be a regular thing happening every few minutes. I don't experience browser crashes for days, so I find it kind of odd that such a relatively rare failure case is considered a good justification for using a process per tab.
I used OneTab for a while, to reduce memory use, but still found Firefox more efficient and more stable with hundreds of tabs, mainly because the vast majority weren't actually loaded until I needed them.
It's not hard to crash Firefox -- just keep loading an infinite page of gifs etc. But once you know that, you don't do it.
Under normal working conditions, I agree: none of the main browsers crashes often enough for it to be a problem.