multifirefox (deprecated)

Install command: 
brew install --cask multifirefox

Name: MultiFirefox

Launcher utility to run multiple versions of Firefox side-by-side

https://davemartorana.com/multifirefox/

Cask JSON API: /api/cask/multifirefox.json

Cask code: multifirefox.rb on GitHub

Current version: 2.3.3

Requirements: macOS >= 10.11

multifirefox is built for Intel macOS and so requires Rosetta 2 to be installed.
You can install Rosetta 2 with:
    softwareupdate --install-rosetta --agree-to-license
Note that it is very difficult to remove Rosetta 2 once it is installed.

Analytics:

Installs (30 days)
multifirefox 7
Installs (90 days)
multifirefox 26
Installs (365 days)
multifirefox 93