Hi David, could you describe a little more about what steps you are trying that are not working for you?

I will comment about the "Clear Data" button on the Settings page: Firefox may resume fetching data into the disk cache within moments of clearing Cached Web Content. To see whether any cookies have been set, try using the "Manage Data" button, which lists cookies and the volume of local storage data, for each site.

Are you using permanent Private Browsing mode (Always use Private Browsing mode; Never Remember History) in case this makes a difference ?

• https://support.mozilla.org/en-US/kb/private-browsing-use-firefox-without-history
• https://support.mozilla.org/en-US/kb/common-myths-about-private-browsing

• Settings -> Privacy & Security
  Firefox will: "Use custom settings for history"
• remove checkmark: [ ] "Always use Private Browsing mode"

Manage Data shows the data usage for regular mode and you can't remove this data when you are in PB mode. You would have to leave PB mode temporarily to be able to remove this data.

• Settings -> Privacy & Security
  Cookies and Site Data: "Manage Data"