These cookies might be stored in sessionstore.jsonlz4 as part of session data. They can also be added by an extension as special opt-out cookies that are send to a website instead of the real cookie. In the past some legacy extensions were known to create such a list, but I don't know whether WebExtensions can also do this.

Start Firefox in Safe Mode to check if one of the extensions ("3-bar" menu button or Tools -> Add-ons -> Extensions) or if hardware acceleration is causing the problem.

• switch to the DEFAULT theme: "3-bar" menu button or Tools -> Add-ons -> Themes
• do NOT click the "Refresh Firefox" button on the Safe Mode start window

• https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-using-safe-mode
• https://support.mozilla.org/en-US/kb/troubleshoot-extensions-themes-to-fix-problems

Thanks for the reply. Alas that didn't solve the query but seeing 'session' did point me towards me in the right direction to a solution.

I always start Firefox with 'Restore previous session' and have so done for a long time.

So I disabled that to have Home pages, restarted Firefox, open a few other sites in tabs, then put back 'Restore...' and restart again.

Now only the cookies from the open tabs are a few seconds old. In addition the 100s of cookies previously a few seconds old, now have a 'Last Used' time before today.

Looks like Firefox was wrongly hanging onto data stored in the 'sessionstore-backups' folder.

One small update. As time goes on with 'Restore...' FF appears to be referencing a history of restarts and/or recent sites viewed as the list of 10sec cookies start to increase again.

One restart with 'Home Pages' restores the cookie Last Used times again.