hello, this is not by intention but a bug that is happening when you use private browsing mode and have set firefox to open windows instead of tabs for a certain kind of website link. this is going to be addressed by mozilla (though it's currently unclear if the fix will make it into firefox 39).

Possible workarounds:

(1) use Options/Preferences > General: "Open new windows in a new tab instead"
(2) Shift+click links to open in a new window with the standard features or use the right-click context menu.

You can modify dom.disable_window_open_feature prefs and change their value to true to prevent a website from disabling toolbars and scroll bars in a pop-up window

• http://kb.mozillazine.org/Prevent_websites_from_disabling_new_window_features

You can open the about:config page via the location/address bar. You can accept the warning and click "I'll be careful" to continue.

• http://kb.mozillazine.org/about:config