diff --git a/README.md b/README.md
index 46e030f6e..b61962887 100644
--- a/README.md
+++ b/README.md
@@ -7,6 +7,11 @@ of websites. OpenWPM is built on top of Firefox, with automation provided
 by Selenium. It includes several hooks for data collection. Check out
 the instrumentation section below for more details.
 
+<span style="color:red">**Warning**</span>: This branch is build on Firefox 52
+ESR, which has been unsupported since June 2018. Crawls with this branch should
+only be run in an isolated environment, as there are almost certainly browser
+exploits for this outdated browser.
+
 Installation
 ------------