Start Firefox in Safe Mode to check if one of the extensions (Firefox/Tools > Add-ons > Extensions) or if hardware acceleration is causing the problem (switch to the DEFAULT theme: Firefox/Tools > Add-ons > Appearance).

Do not click the Reset button on the Safe mode start window or otherwise make changes.

   https://support.mozilla.org/kb/Safe+Mode
   https://support.mozilla.org/kb/Troubleshooting+extensions+and+themes

Thanks for suggestion.

Have tried this but no change.

Bob

Better reset your Firefox Over time Firefox can develop problems like slowness, crashing, unwanted toolbars and more. Troubleshooting and fixing these issues can be difficult and time consuming. The Reset Firefox feature can fix many issues by restoring Firefox to its factory default state while saving your essential information.

https://support.mozilla.org/en-US/kb/reset-firefox-easily-fix-most-problems#w_what-does-the-reset-feature-do