These are Windows screen savers, or something specific to Firefox?

If this is a problem in Firefox, one possible culprit would be an extension. You can view, disable, and often remove unwanted or unknown extensions on the Add-ons page. Either:

• Ctrl+Shift+a (Mac: Command+Shift+a)
• "3-bar" menu button (or Tools menu) > Add-ons
• type or paste about:addons in the address bar and press Enter/Return

In the left column of the Add-ons page, click Extensions. On the right side, find the "Manage Your Extensions" heading.

If there is at least one extension before the next heading -- "Recommended Extensions" -- please continue:

Then cast a critical eye over the list below that heading. Any extensions Firefox installs for built-in features are hidden from this page, so everything listed here is your choice (and your responsibility) to manage. Anything Bing-related, screensaver/wallpaper-ish, suspicious or that you just do not remember installing or why? If in doubt, disable (or remove). For your privacy and security, don't let mystery programs linger here.

Any improvement?

Some screen savers have their own option menu. Check for this.

Thanks for the suggestions. Only add on is adblock. No change. Goes to bing images when mouse moved, then normal MZ when this is closed. Tried to look in start menu but wasn't prepared to risk changing anything.

I'm not using a screen saver (see attached "Screen Saver Settings" dialog), but does yours show there that you are set to use a screen saver?

Thank you.. Eventually found the control in bing. (Mostly by luck and perseverance)