Thank goodness this Community Forum gave me the answer I needed - someone suggested Safari for Mac users and that works, hooray!
I am compelled to add that the ONLY reason I have Firefox at all is because of an earlier problem with the Auctiva uploader. Safari was incompatible so I had to use Firefox.
It would've been nice if Auctiva told us they were changing the uploader and perhaps posted their "solutions" so we didn't lose so much valuable time fighting with it.