Okay, imagine my surprise when this evening, without warning, my copy of firefox started doing EXACTLY what my friend's was doing. I had not really changed anything that would have affected that, but...

Further inquiries with others who have firefox discovered that theirs did it too.

Then, a breakthrough. If left long enough, the file actually is getting uploaded. In other words, it is, actually, uploading the file, it's just the progress bar that is wonky.

And wasn't wonky this morning when I uploaded to the same site.

This seems to indicate that some automatic update has changed something.

I have contacted Firefox to see if they might know.