The update to this version, with Firefox Developer Edition, breaks the browser under macOS. If the app is in the dock, you need to option-click the icon, then click the button in the window that appears that starts with 'Refresh' to un-break it.
1. I hadn't seen that guide, but there is some issue where Catalina thinks that the command line tools installed are up to date, but `brew doctor` reports otherwise and you are forced to manually download the command line tools, just like in that guide.
2. I tried zsh briefly, then just used the bash that you can install via homebrew.