So I restarted my computer and ran CCleaner (many times). It skips all cleaning work for thunderbird including cookies. I had a look with task manager and nothing from Thunderbird is running. Any ideas on how to get CCleaner to delete those files.
If it's skipping then something related is running in the background even though Thunderbird itself is closed.
See the section "For other apps" of this post which is about turning off background running for apps.
Try using it to turn off background running for Thunderbird:
Thank you for the reply and advice. That sorted it out.
Cheers