Thank you for your prompt query.
Based on the shared details, I suspect that the frame uses the so called "cross-domain" requests and thus cannot be automated in Firefox due to the limitation about CORS
(cross-domain resource sharing). As of our perspective, this is a known limitation for Firefox automation. So, please excuse us for the inconvenience you couldn't find any information on the topic - I will store this feedback for our future releases.
To double check if my assumption is correct, you can check the source of the frame - if its domain is different than the automated one, then this frame uses a cross-domain request.
I hope the above information will be helpful for you. Though, in case of further questions, please get back to me. In the case you are not sure about the type of frame you are testing, please provide a sample test which can demonstrate the described issue.
Thank you in advance.