Fix internal assertion encountered when testing with jsdom.

.changeset/hot-turkeys-promise.md

@@ -0,0 +1,5 @@
+---
+"@firebase/firestore": patch
+---
+
+Fix internal assertion due to Buffer value not evaluating to instanceof Uint8Array, encountered when testing with jsdom.

packages/firestore/src/remote/serializer.ts

@@ -262,8 +262,14 @@ export function fromBytes(
     return ByteString.fromBase64String(value ? value : '');
   } else {
     hardAssert(
-      value === undefined || value instanceof Uint8Array,
-      'value must be undefined or Uint8Array'
+      value === undefined ||
+        // The Buffer interface extends the Uint8Array interface, however
+        // the Uint8ArrayConstructor is not in the prototype chain of
+        // Buffer, so we have to test `value instanceof Buffer` and
+        // `value instanceof Uint8Array`.
+        value instanceof Buffer ||
+        value instanceof Uint8Array,
+      'value must be undefined, Buffer, or Uint8Array'
     );
     return ByteString.fromUint8Array(value ? value : new Uint8Array());
   }