6 is in beta now. Nightlies were 7.0a a few days ago, but if you update.... yep, 8.0a1
PS. Nightly has been incredibly stable on my Mac. I actually use it in production, for no other reason than I was testing it and have no reason to get an old release since it's never crashed and is incredibly fast.
Actually most browsers' alphas are very stable these days. At home I run Opera Next, Chrome Canary and Firefox Nightly and I haven't had any problems for months.
Sometimes some websites are not shown properly in one of those but all it takes is to switch to another browser.
I run Aurora and use the Add-on Compatibility Reporter[1] to disable version checks and run any add-on I want. For most of my add-ons this works fine, but if there is a significant API change an add-on might not work correctly. But if you were scared of that you wouldn't be using Nightly or Aurora.
PS. Nightly has been incredibly stable on my Mac. I actually use it in production, for no other reason than I was testing it and have no reason to get an old release since it's never crashed and is incredibly fast.