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.

Thanks, Tony. I am indeed using tabmixplus. It sounds like "protect" is what I want -- wanting to keep from accidentally closing an app tab, which I guess is prevented within native firefox, but with the close current tab button on tabmix plus, I have occasionally closed an app tab by error.

BTW, it appears that "lock" does not work as described in FF 4. I have an app tab for Huffington post, which I both protected and locked. I can't close it, but it freely navigates to links within huffington post that are not set to open in a new tab.

I can not replicate the problem with locked tabs, all links I tried opened in a new tab. I am using the latest development version so it may be something that has been fixed. You can try the development version, it is available from http://tmp.garyr.net/forum/viewtopic.php?t=10888

If you still get the problem with locked tabs, you can report it to the Tab Mix Plus developer - http://tmp.garyr.net/forum/viewforum.php?f=2