I am sorry to hear you are running into this problem. In order to investigate why you are unable to find the element in Firefox we need to reproduce the issue locally. If your app is publicly accessible then you can attach a copy of your test so I can troubleshoot it directly. As an alternative, you can also record a fiddler trace using FiddlerCap
and provide it to me so I can run the test against it. Basically with that trace I'll try to "simulate" your app in order to reproduce the problem.
If you deem that information too sensitive for this forum, you can create a support ticket and attach it there. Thanks for providing the information we need to best assist you.
the Telerik team