--- a/testing/marionette/addon.js
+++ b/testing/marionette/addon.js
@@ -83,18 +83,24 @@ async function installAddon(file) {
  *
  * @return {Promise.<string>}
  *     Addon ID.
  *
  * @throws {UnknownError}
  *     If there is a problem installing the addon.
  */
 addon.install = async function(path, temporary = false) {
-  let file = new FileUtils.File(path);
   let addon;
+  let file;
+
+  try {
+    file = new FileUtils.File(path);
+  } catch (e) {
+    throw new UnknownError(`${path} is not an absolute path.`);
+  }
 
   if (!file.exists()) {
     throw new UnknownError(`Could not find add-on at '${path}'`);
   }
 
   try {
     if (temporary) {
       addon = await AddonManager.installTemporaryAddon(file);

--- a/testing/marionette/harness/marionette_harness/tests/unit/test_addons.py
+++ b/testing/marionette/harness/marionette_harness/tests/unit/test_addons.py
@@ -92,8 +92,12 @@ class TestAddons(MarionetteTestCase):
         with self.assertRaises(AddonInstallException):
             self.addons.install(addon_path)
 
     def test_install_nonexistent_addon(self):
         addon_path = os.path.join(here, "does-not-exist.xpi")
 
         with self.assertRaisesRegexp(AddonInstallException, "Could not find add-on at"):
             self.addons.install(addon_path)
+
+    def test_install_with_relative_path(self):
+        with self.assertRaisesRegexp(AddonInstallException, "is not an absolute path."):
+            self.addons.install('webextension.xpi')