Try clicking the X to the right of the URL/address to cancel the download, then hitting the circular arrow to restart the download. Try that a couple of times. Once downloaded into the process space of FF, you should have the”Open With Different Viewer” button to the top-right of the document window in FF.
The bug (assuming it’s a bug) will no doubt be patched soon. I experienced the prob too!
Firefox quality is going downhill. Mozilla must be hiring aspiring Microsoft programmers.