If your Steam won’t open on Mac, just like mine did a minute ago, here’s a quick solution for you that worked for me right away.
1. Open Finder
2. Click “Go” in the top menu bar
3. Click “Library” in the “Go” menu
4. Find Steam folder, and click on it
5. Delete a file named “registry.vdf” (don’t worry, Steam will create it again when you relaunch it)
Next thing I knew when I did this, is Steam asking wether to reopen previous windows. I clicked (hell) “No”.
Hope this was helpful. Let me know if it was/is.