The fontFamily expects a comma separated list apparently, and families with spaces have to be quoted within the JSON string.

VS-Code-Instructions.md

@@ -6,7 +6,7 @@ Add `"editor.fontLigatures": true` to `settings.json`:
 
 To open `settings.json`, from the `File` menu choose `Preferences`, `User Settings`. Then paste this:
 ```json
-    "editor.fontFamily": "Fira Code",
+    "editor.fontFamily": "'Fira Code'",
     "editor.fontSize": 14,
     "editor.fontLigatures": true
 ```