diff --git a/tools/test-js-with-puppeteer b/tools/test-js-with-puppeteer
index 5445f93007..41e4ce7b8d 100755
--- a/tools/test-js-with-puppeteer
+++ b/tools/test-js-with-puppeteer
@@ -15,6 +15,10 @@ ZULIP_PATH = os.path.dirname(os.path.dirname(os.path.abspath(__file__)))
 # watch mode.
 os.environ["PUPPETEER_TESTS"] = "1"
 
+# The locale can have impact how Firefox does locale-aware sorting,
+# which we do verify in some tests.
+os.environ["LC_ALL"] = "en_US.UTF-8"
+
 os.environ["CHROMIUM_EXECUTABLE"] = os.path.join(ZULIP_PATH, "node_modules/.bin/chromium")
 os.environ.pop("http_proxy", "")
 os.environ.pop("https_proxy", "")