This is going to require a change in the software. You can tell them the following:

Unless you prevent the default with event.preventDefault(), the browser right-click menu will show. An example of this is http://jsfiddle.net/7xry5rbe/.

Also, all browsers do this the same way - this will happen outside of Firefox as well.

First, you can press and release the Alt key to clear Firefox's context menu and reveal any menu the site has generated behind it.

Second, there is a setting to make sure Firefox's menu is always shown even if the site tries to replace it. Most users have not turned on that setting, but if you have been using Firefox for many years, back to when there were Advanced script settings available through the Options dialog, you might have done it back then. You could check here:

(1) In a new tab, type or paste about:config in the address bar and press Enter. Click the button promising to be careful.

(2) In the search box above the list, type or paste contex and pause while the list is filtered

(3) If the dom.event.contextmenu.enabled preference is bolded and "user set" to false, double-click it to restore the default value of true.

This is likely a problem with the dom.event.contextmenu.enabled pref that jscher2000 mentions as the third suggestion in the above post, so make sure that this pref is set to true.

Note that there are dom.disable_window* prefs to prevent websites from hiding UI items that are useful.

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

There are a few more dom.event.* and dom.disable* prefs that are best left to their defaults.

dom.allow_scripts_to_close_windows  [false]
dom.disable_beforeunload  [false]
dom.disable_image_src_set  [false]
dom.event.clipboardevents.enabled  [true]
dom.event.contextmenu.enabled  [true]

jscher2000 said

First, you can press and release the Alt key to clear Firefox's context menu and reveal any menu the site has generated behind it. Second, there is a setting to make sure Firefox's menu is always shown even if the site tries to replace it. Most users have not turned on that setting, but if you have been using Firefox for many years, back to when there were Advanced script settings available through the Options dialog, you might have done it back then. You could check here: (1) In a new tab, type or paste about:config in the address bar and press Enter. Click the button promising to be careful. (2) In the search box above the list, type or paste contex and pause while the list is filtered (3) If the dom.event.contextmenu.enabled preference is bolded and "user set" to false, double-click it to restore the default value of true.

Thanks for the info, the setting is indeed set to "false" but locked (in italic) ... I don't seem to have permission to unlock so will ask administrators,

Are more prefs locked?

You can check that by clicking the Status column to sort by its content.