diff --git a/rb/CHANGES b/rb/CHANGES
index 7d93eee494cf0..c989013b66eb9 100644
--- a/rb/CHANGES
+++ b/rb/CHANGES
@@ -3,6 +3,7 @@
 
 Ruby:
   * Removed dependency on "multi_json" (issue 1632)
+  * Properly handle namespaces in install manifest of Firefox add-ons (issue 1143)
 
 2.52.0 (2016-02-12)
 ===================
diff --git a/rb/lib/selenium/webdriver/firefox/extension.rb b/rb/lib/selenium/webdriver/firefox/extension.rb
index d52be4d1cab3a..c2175c3dddb06 100644
--- a/rb/lib/selenium/webdriver/firefox/extension.rb
+++ b/rb/lib/selenium/webdriver/firefox/extension.rb
@@ -21,8 +21,13 @@ module Selenium
   module WebDriver
     module Firefox
 
+      #
       # @api private
+      #
+
       class Extension
+        NAMESPACE = 'http://www.mozilla.org/2004/em-rdf#'
+
         def initialize(path)
           unless File.exist?(path)
             raise Error::WebDriverError, "could not find extension at #{path.inspect}"
@@ -61,20 +66,17 @@ def create_root
         def read_id_from_install_rdf(directory)
           rdf_path = File.join(directory, "install.rdf")
           doc = REXML::Document.new(File.read(rdf_path))
+          namespace = doc.root.namespaces.key(NAMESPACE)
 
-          id_node = REXML::XPath.first(doc, "//em:id")
-
-          if id_node
-            id_node.text
-          else
-            attr_node = REXML::XPath.first(doc, "//@em:id")
+          if namespace
+            id_node = REXML::XPath.first(doc, "//#{namespace}:id")
+            return id_node.text if id_node
 
-            if attr_node.nil?
-              raise Error::WebDriverError, "cannot locate extension id in #{rdf_path}"
-            end
-
-            attr_node.value
+            attr_node = REXML::XPath.first(doc, "//@#{namespace}:id")
+            return attr_node.value if attr_node
           end
+
+          raise Error::WebDriverError, "cannot locate extension id in #{rdf_path}"
         end
 
       end # Extension
diff --git a/rb/spec/unit/selenium/webdriver/firefox/extension_spec.rb b/rb/spec/unit/selenium/webdriver/firefox/extension_spec.rb
index 0adbf45a135e0..f5fcaf521af43 100644
--- a/rb/spec/unit/selenium/webdriver/firefox/extension_spec.rb
+++ b/rb/spec/unit/selenium/webdriver/firefox/extension_spec.rb
@@ -60,6 +60,19 @@ def ext.read_id(dir); read_id_from_install_rdf(dir); end
           expect(extension.read_id('/foo')).to eq('{f5198635-4eb3-47a5-b6a5-366b15cd2107}')
         end
 
+        it 'finds the rdf extension id regardless of namespace' do
+          allow(File).to receive(:read).with('/foo/install.rdf').and_return <<-XML
+            <?xml version="1.0"?>
+            <r:RDF xmlns:r="http://www.w3.org/1999/02/22-rdf-syntax-ns#" xmlns="http://www.mozilla.org/2004/em-rdf#">
+                <r:Description about="urn:mozilla:install-manifest">
+                    <id>{f5198635-4eb3-47a5-b6a5-366b15cd2107}</id>
+                </r:Description>
+            </r:RDF>
+          XML
+
+          expect(extension.read_id('/foo')).to eq('{f5198635-4eb3-47a5-b6a5-366b15cd2107}')
+        end
+
         it 'raises if the node id is not found' do
           allow(File).to receive(:read).with('/foo/install.rdf').and_return <<-XML
             <?xml version="1.0"?>
@@ -68,10 +81,8 @@ def ext.read_id(dir); read_id_from_install_rdf(dir); end
 
           expect { extension.read_id('/foo') }.to raise_error(Error::WebDriverError)
         end
-
       end
 
     end # Firefox
   end # WebDriver
 end # Selenium
-