Mac El Capitan, FirefoxDeveloperEdition (which iirc is basically equivalent to Firefox Beta), not many addons (Tab Groups, uBlock Origin, No More 404s).
Even more impressive, the Nightly versions of both Chrome and Firefox are (almost always) stable enough for daily use. I use them as my everyday browsers at work, and they probably crash / have bugs about 1 day out of every 2 months.
Agreed. I started using Nightly — instead of my usually Beta¹ — for the up-to-date GTK+ 3.2x compatibility, but I like some of the pre-release features, and it almost never crashes on me
1: On Ubuntu you can use their Firefox Beta PPA² as long as you don't mind that it replaces release-version Firefox.
I started using Nightly for kicks and giggle around 6 months ago and my about:crashes shows me only 6 crashes (none of which were able to kill anything more than a tab (and in all the cases, the offending website was YouTube)).
Nightly is amazingly good even if it is built from the HEAD of the trunk (branch in git terms).
The Developer Edition replaced Aurora, which was the alpha release, one version between the beta and Nightly.