firefox cannot download after jessie->stretch

[Frank Miles <fpm@u.washington.edu>]

firefox/iceweasel has worked just fine for years

I just upgraded my home jessie machine to stretch.  That went very 
smoothly, so far only one exception.  I can no longer download anything
with this browser.  I've reinstalled, cleared out my profile, run in
'safe' mode (no add-ons such as no-script).  I've tried other 
destinations, including "ask me" (it never asks).  I only get the
truncated error message:
   "could not be saved, because you cannot change the contents of that 
folder."

Turned off my firewall - no change.  Tried downloading from other sources,
no change.

Epiphany downloads the same files with no problem.

Has anyone else had this problem recently ?  Looking at the Debian bug 
reports I saw a year-old report that looked kind of similar.

  Thanks for any insights!
    -Frank