This could happen if your Firefox is set to use automatic private browsing mode. In that case, all windows look the same. Could you check your setting here:

• "3-bar" menu button (or Tools) > Options

In the left column, click Privacy.

If the selector on the right side says "Firefox will: Never remember history" change that to "Firefox will: Use custom settings for history"

Then check: is there a checkmark in the box for "Always use private browsing mode"?

If Yes: What you are seeing is normal. Automatic private browsing only has private windows and they are not marked specially.

If No: Firefox seems to be broken!

jscher2000 thanks a lot!

Note that you can use an extension to get Private Browsing mode support per tab.

• https://addons.mozilla.org/firefox/addon/private-tab/
• Private Browsing mode tabs get a dashed underline
• You can toggle Private Browsing mode out-of- via the right-click context menu of a tab