Those are not standard Firefox entries, but can be added by extensions such as Tab Mix Plus.

The following is taken from the Tab Mix Plus user manual - http://tmp.garyr.net/help

Locked tab: A tab that is prevented from navigating to another page or URL. Your tab is locked on the current page or URL. Nothing new can load in the tab, but you can refresh the same page. All links will open in a new tab (link, history, bookmark, etc.). Notice that the close button is available, so you can close a locked tab.

Protected tab: A tab that is prevented from closing. You can load other sites in the tab and browse to other URLs in the tab. Notice that the close button is missing from the right side of the tab, so you cannot close the tab. If you try to close Firefox, you will get a warning message about closing the protected tab.

You are right. I added Tab Mix Plus ages ago and I guess I didn't expect it to make the leap to 4 like it did.

Thank you very much for your assistance.