Mac El Capitan, FirefoxDeveloperEdition (which iirc is basically equivalent to Firefox Beta), not many addons (Tab Groups, uBlock Origin, No More 404s).
The Developer Edition replaced Aurora, which was the alpha release, one version between the beta and Nightly.
Credit to them though, Aurora was surprisingly stable for most of its lifetime, despite not even being a beta.
1: On Ubuntu you can use their Firefox Beta PPA² as long as you don't mind that it replaces release-version Firefox.
2: https://launchpad.net/~mozillateam/+archive/ubuntu/firefox-n...
Nightly is amazingly good even if it is built from the HEAD of the trunk (branch in git terms).