diff --git a/doc/internals/contributing.rst b/doc/internals/contributing.rst
index b62746806..35f17574d 100644
--- a/doc/internals/contributing.rst
+++ b/doc/internals/contributing.rst
@@ -197,6 +197,14 @@ To run JavaScript tests, use ``npm``::
     npm install
     npm run test
 
+.. tip::
+
+  `karma` requires a Firefox binary to use as a test browser.
+
+  For Unix-based systems, you can specify the path to the Firefox binary using::
+
+    FIREFOX_BIN="/Applications/Firefox.app/Contents/MacOS/firefox" npm test
+
 New unit tests should be included in the ``tests`` directory where
 necessary: