Evil Firefox Error -- "A parameter or an operation is not supported by the underlying object"

Okay, so after of hours of testing (and great discussion from @Dave and @danronmoon, I've finally figured out what's going on.

The CORS (Cross-Domain Resource Sharing) calls I was making were set to 'async: false' -- (which I realize I did not include in my original post, as I thought it was inconsequential) this, seems to operate fine in all browsers except Firefox, where jQuery will bark at you and your ajax call will fail.

Thank you all for your help and I hope this helps someone else!

Since this is the first duckduckgo result for InvalidAccessError: A parameter or an operation is not supported by the underlying object I will add another source for this.

If you deal with such error when doing iframe/window actions, then you're probably prevented by the iframe's sandbox attribute (see https://html.spec.whatwg.org/multipage/iframe-embed-object.html#attr-iframe-sandbox ) even when being on the same origin.

In my case, an iframe was trying to do a window.top.location.href = ... after a form submission success. The allow-top-navigation sandbox option is mandatory to do so. Funny thing, this sandbox option is not mandatory to reload the top browsing context... it's only required for navigating in it.

For me, I was using WebSockets and called WebSocket.close(1001). It doesn't like my status code. Changing it to 1000 or not specifying a code (default 1005) works just fine.