homebrew-cask-versions
Softwarehomebrew-cask-versions lets you install previous or beta versions of popular GUI tools that aren’t in mainline homebrew cask.
$ brew tap homebrew/cask-versions
$ brew cask install 1password6
Browsing the casks in the repo, I can see they have the latest testing builds for Firefox, older versions of VMware Fusion, and other good stuff.