fix: add missing firebase base class

diagrams/firebase/__init__.py

@@ -0,0 +1,12 @@
+"""
+Firebase provides a set of services for Firebase provider.
+"""
+
+from diagrams import Node
+
+
+class _Firebase(Node):
+    _provider = "firebase"
+    _icon_dir = "resources/firebase"
+
+    fontcolor = "#ffffff"