How to reset MacOS QGIS to defaults?

Try deleting org.qgis.QGIS.plist in the Library/Preferences directory of your home directory. You might want to make a backup copy of it first just in case.

QGIS on Mac & *Nix also stores some information inside of a folder (invisible on Macs) inside of your home directory. The folder is located at ~/.qgis . You may need to delete one or more files from this folder.

You can access this folder in Finder by clicking the Go menu and choosing Go to Folder. Type in ~/.qgis and click Go.

Note: Backing up is also advised prior to making any changes.