Unless others can reproduce the same error that makes diagnosing it hard to do.

So what problems do you suggest might be the causation?

Just to add I have Kaspersky running all the time and I have used Ess to check for infected files, just in case Kaspersky missed any, however there are no reported infected files.

Or is the vagaries of Win10!

Please uninstall Firefox. Then Delete the Mozilla Firefox Folders in C:\Program Files and C:\Program Files(x86) Then restart system. Then run Windows Disk Cleanup. (Note: this should be pinned and ran weekly, if have not done below before you will find there is 10's of gigs) Then run it again and click the button that says Cleanup System Files. Note: your Firefox Profile is saved. But you should make a back up before you do : https://support.mozilla.org/en-US/kb/back-and-restore-information-firefox-profiles also https://support.mozilla.org/en-US/kb/export-firefox-bookmarks-to-backup-or-transfer

Reinstall with Current Release Firefox 57.0.2 with a Full Version Installer https://www.mozilla.org/firefox/all/

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

Thank you for this and I have followed your guidance to the letter. Incidently I do "Win Disc Cleanup" but not weekly more like twice a month so I will now follow your advice.

I will await the next updates and see if this has resolved the issue, but even so I am grateful for your help.

So what O/S version and x86 or x64 are you running along with what FF version from before x86 or x64 and current FF of x86 or x64 are you using then?

O/S Windows 10 Pro version 1709 64bit. FF version 57.0.2 64bit current (have no ideas what the version was before last weeks update).