Please consider turning on send info to Mozilla before posting a question. This helps us help you. As I have no info from your system that would help see things that cause issues.

All I have is Win 10, Firefox version 56.0 & not responding

Sorta like making a appointment to get your car worked on but show up with out your car.

So since it says you are running version 56 I will get you to uninstall that please ( your profile with passwords, bookmarks are saved) and install the current version 56.0.2 from a Full Version Installer. https://www.mozilla.org/firefox/all/

Also please make sure your video card drivers are up to date.

Please also follow this : https://support.mozilla.org/en-US/kb/troubleshoot-extensions-themes-to-fix-problems?redirectlocale=en-US&redirectslug=Troubleshooting+extensions+and+themes A bad or corrupted Extension can cause issues that make Firefox get the error message not responding.

I am hoping this fixes the issue you have.

Please let us know if this solved your issue or if need further assistance.

When I checked I discovered I'm already running version 56.0.2. Perhaps, since I have Firefox updates automatically installed, it updated between the time I posted my question and the time you replied? Anyway, while I was in Options, on the Performance tab, I unchecked "use recommended settings..." and checked "use hardware acceleration". I don't know which of these worked, the new version or the hardware acceleration, but so far I haven't had the "not responding" issue for the entire day. I'm marking your suggestion as "helped" and the issue "resolved" because without your suggestion I wouldn't have thought to look at the Performance options, Thanks!