diff --git a/src/js/util/connection-doctor.js b/src/js/util/connection-doctor.js
index 6903844..43e047a 100644
--- a/src/js/util/connection-doctor.js
+++ b/src/js/util/connection-doctor.js
@@ -162,12 +162,18 @@ define(function(require) {
 
         socket.ondata = function() {}; // we don't actually care about the data
 
-        socket.oncert = function() {
-            if (options.ca) {
-                // the certificate we already have is outdated
-                error = createError(TLS_WRONG_CERT, strings.connDocTlsWrongCert.replace('{0}', host));
-            }
-        };
+        // [WO-625] Mozilla forbids extensions to the TCPSocket object, 
+        // throws an exception when assigned unexpected callback functions.
+        // The exception can be safely ignored since we need the callback
+        // for the other shims
+        try {
+            socket.oncert = function() {
+                if (options.ca) {
+                    // the certificate we already have is outdated
+                    error = createError(TLS_WRONG_CERT, strings.connDocTlsWrongCert.replace('{0}', host));
+                }
+            };
+        } catch (e) {}
 
         socket.onerror = function(e) {
             if (!error) {
diff --git a/test/unit/connection-doctor-test.js b/test/unit/connection-doctor-test.js
index 97f2ed9..1ffeb08 100644
--- a/test/unit/connection-doctor-test.js
+++ b/test/unit/connection-doctor-test.js
@@ -23,6 +23,7 @@ define(function(require) {
                     this.onclose();
                 }
             };
+
             imapStub = sinon.createStubInstance(ImapClient);
             smtpStub = sinon.createStubInstance(SmtpClient);
 
@@ -95,6 +96,29 @@ define(function(require) {
                 socketStub.onopen();
             });
 
+            it('should catch Mozilla TCPSocket exception', function(done) {
+                // Mozilla forbids extensions to the TCPSocket object
+                Object.defineProperty(socketStub, 'oncert', {
+                    set: function() {
+                        throw 'Mozilla specific behavior';
+                    }
+                });
+
+                doctor._checkReachable(credentials.imap, function(error) {
+                    expect(error).to.not.exist;
+                    expect(TCPSocket.open.calledOnce).to.be.true;
+                    expect(TCPSocket.open.calledWith(credentials.imap.host, credentials.imap.port, {
+                        binaryType: 'arraybuffer',
+                        useSecureTransport: credentials.imap.secure,
+                        ca: credentials.imap.ca
+                    })).to.be.true;
+
+                    done();
+                });
+
+                socketStub.onopen();
+            });
+
             it('should fail w/ wrong cert', function(done) {
                 doctor._checkReachable(credentials.imap, function(error) {
                     expect(error).to.exist;