Hi EricG, a common method sites use to guess that Firefox is in private browsing mode is to test whether they can make persistent saves to a database using the IndexedDB feature. So possibly IndexedDB is disabled in your Firefox for some reason.

Your Firefox identified itself as version 60 on Windows 7. Is that true, or are you using the "resistFingerprinting" preference?

That is true Windows 7 version 60 - I do not see how to enable IndexedDB in any of the options pulldowns - is there a way to check if is disabled and if so how to enable it?

As a quick test of whether IndexedDB is enabled, does this little web application work for you:

http://mdn.github.io/to-do-notifications/

You can check the Web Console for possible storage related messages.

You can also make sure that some content isn't blocked. When specific content is blocked then website might assume that PB mode is active.

If you use extensions ("3-bar" menu button or Tools -> Add-ons -> Extensions) that can block content (Adblock Plus, NoScript, Disconnect, Ghostery, Privacy Badger, uBlock Origin) always make sure such extensions do not block content.

• make sure your extensions and filters are updated to the latest version
• https://support.mozilla.org/en-US/kb/troubleshoot-extensions-themes-to-fix-problems

Firefox shows the shield icon at the left end of the location/address bar in case Content Blocking is blocking content.

• click the shield icon for more details and possibly disable the protection

You can check the Web Console about what content is blocked

• "3-bar" menu button or Tools -> Web Developer
• https://support.mozilla.org/en-US/kb/content-blocking