Make sure you are not blocking content.

Start Firefox in Safe Mode {web link}

A small dialog should appear. Click Start In Safe Mode (not Refresh). Is the problem still there?

One issue that seems more common lately is Firefox's Tracking Protection feature. When it is blocking content in a page, a shield icon will appear at the left end of the address bar next to the padlock icon. This article has more info on managing this feature: Tracking Protection {web link}

Thanks for the response - I restarted in Safe Mode as instructed but no change in problem - videos appear to start for a split second and then freeze

Start your Computer in safe mode with network support. Then start Firefox. Try Secure websites. Is the problem still there?

http://encyclopedia2.thefreedictionary.com/Linux+Safe+Mode Starting Any Computer In Safe Mode; Free Online Encyclopedia

Thank you for your efforts - the things to do as suggested have not worked however - I just got a Windows update (KB4489899) to version Windows 10 Ver 1809 and the problem seems to be resolved - The problem only seemed to affect Firefox but is now back to normal - thanks again.

That's good to hear. Well done. Please flag your last post as Solved Problem so others will know.

Go to the Firefox 3 Bar Menu > Help > Troubleshooting Information Page and take a look in the Accessibility section if accessibility is set to "true" there, go to the Firefox 3 Bar Menu --> Options --> Privacy & Security panel and under Permissions check "Prevent Accessibility Services from accessing your browser" and restart Firefox. I used to have the same problem and the issue was with a little app called Unchecky.