Maybe toggling this pref on the about:config page will work.

• dom.event.contextmenu.enabled = false

You can open the about:config page via the location/address bar. You can accept the warning and click "I accept the risk!" to continue.

• https://support.mozilla.org/en-US/kb/about-config-editor-firefox

Thanks, but doesn't help. I believe that pref stops sites from messing with context menus (usually right-click menus.) I do figure it's a pref somewhere that I can't find.

Is this about hiding toolbars like the Firefox Menu bar in a pop-up window ?

There are various dom.disable_window_open_feature prefs to prevent websites from hiding specific content that you can set to true to prevent a website from disabling this specific feature.

• dom.disable_window_open_feature.menubar = true

Yes, that is exactly what I need. (I also see a list of similar items one can keep on the new windows listed at http://kb.mozillazine.org/About:config_entries , thanks to your guidance to get me on the right track. )

Thanks muchly!

You're welcome.

See also:

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