I had several more crashes of FF on trying to restart it. The other crashes pointed to different web pages, so it probably is a plug-in and not a particular page.

The only way I could get it to come up was to have it load only my (text link) home page (no other external tabs). At that point I could disable the "20-20 3D Viewer 3.0.31.0" in Addons -> Extensions.

But now my friendly 150-200 kB background download is back. Grr.

When I get a chance, I'll go through the diagnoses you folks suggested. For now, I'll close FF and see if Chrome or Safari has the same issues.

Thanks.

Cheers,
Scott.