12.0.3 browser is causing strange activity on motion GIFs on Win10

I have been using the Tor browser to save motion GIFs for a while. During the downloads,
the icon showing in the destination directory has always been the rainbow-colored raindrop
from 3DPaint. But when the download finishes, the icon changes to a miniature image.
Right-clicking on this image to check properties has always shown a file between 10 and 40
megabytes which plays normally in XnView from the file on the destination drive.
…After 12.0.3 installation, “Save image as GIF” now only saves a single image of a few
hundred kilobytes, according to a properties check.
…Is there a setting in about:config that I need to correct this? Did an accidental startup of
3DPaint attach itself to a file extension and change it to cause this? Can change it back?