--- a/testing/geckodriver/src/prefs.rs
+++ b/testing/geckodriver/src/prefs.rs
@@ -3,17 +3,17 @@ use mozprofile::preferences::Pref;
 // ALL CHANGES TO THIS FILE MUST HAVE REVIEW FROM A GECKODRIVER PEER!
 //
 // The Marionette Python client is used out-of-tree with release
 // channel builds of Firefox.  Removing a preference from this file
 // will cause regressions, so please be careful and get review from
 // a Testing :: Marionette peer before you make any changes to this file.
 
 lazy_static! {
-    pub static ref DEFAULT: [(&'static str, Pref); 52] = [
+    pub static ref DEFAULT: [(&'static str, Pref); 53] = [
         // Make sure Shield doesn't hit the network.
         ("app.normandy.api_url", Pref::new("")),
 
         // Disable automatic downloading of new releases
         ("app.update.auto", Pref::new(false)),
 
         // Disable automatically upgrading Firefox
         ("app.update.enabled", Pref::new(false)),
@@ -88,16 +88,20 @@ lazy_static! {
 
         // Disable intalling any distribution extensions or add-ons
         ("extensions.installDistroAddons", Pref::new(false)),
 
         // Make sure Shield doesn't hit the network.
         // TODO: Remove once minimum supported Firefox release is 60.
         ("extensions.shield-recipe-client.api_url", Pref::new("")),
 
+        // Disable extensions compatibility dialogue.
+        // TODO: Remove once minimum supported Firefox release is 61.
+        ("extensions.showMismatchUI", Pref::new(false)),
+
         // Turn off extension updates so they do not bother tests
         ("extensions.update.enabled", Pref::new(false)),
         ("extensions.update.notifyUser", Pref::new(false)),
 
         // Allow the application to have focus even it runs in the
         // background
         ("focusmanager.testmode", Pref::new(true)),
 

--- a/testing/marionette/client/marionette_driver/geckoinstance.py
+++ b/testing/marionette/client/marionette_driver/geckoinstance.py
@@ -66,16 +66,18 @@ class GeckoInstance(object):
         "extensions.enabledScopes": 5,
         # Disable metadata caching for installed add-ons by default
         "extensions.getAddons.cache.enabled": False,
         # Disable intalling any distribution add-ons
         "extensions.installDistroAddons": False,
         # Make sure Shield doesn't hit the network.
         # Removed in Firefox 60.
         "extensions.shield-recipe-client.api_url": "",
+        # Disable extensions compatibility dialogue.
+        # Removed in Firefox 61.
         "extensions.showMismatchUI": False,
         # Turn off extension updates so they don't bother tests
         "extensions.update.enabled": False,
         "extensions.update.notifyUser": False,
         # Make sure opening about:addons won"t hit the network
         "extensions.webservice.discoverURL": "http://%(server)s/dummy/discoveryURL",
 
         # Allow the application to have focus even it runs in the background