Is there no Tab section in the Options? That is where the setting should be (unless that was removed).

It's still there.

Open new windows in a new tab instead doesn't always work when a JavaScript hyperlink is used on the webpage. It forces a popup window to open, sometimes for security reasons. Like a "Login" button on a banking or "payments" type website - where I see that the most. A single tab in a new window is more secure then an existing window which may have other tabs already open. Sorry, I don't know how to block that from opening a new window, beyond using a {middle-click} or {ctrl-click} to open those JavaScript hyperlinks, when you encounter one.

I use this UserStyle - https://userstyles.org/styles/233/change-cursor-on-some-links-with-js - to "tell me" that a hyperlink is one using JavaScipt by changing the cursor appearance.

See also:

• http://kb.mozillazine.org/browser.link.open_newwindow
• 1: current tab; 2:new window; 3:new tab;

For links opened via JavaScript you can look at this pref:

• http://kb.mozillazine.org/browser.link.open_newwindow.restriction

Your System Details list shows that you have a user.js file in the profile folder to initialize prefs each time Firefox starts.

The user.js file is only present if you or other software has created this file and normally it wouldn't be there. You can check its content with a plain text editor (right-click: Open with) if you didn't create this file yourself.

The user.js file is read each time Firefox is started and initializes preferences to the value specified in this file, so preferences set via user.js can only be changed temporarily for the current session.

You can delete possible user.js and numbered prefs-##.js files and rename (or delete) the prefs.js file to reset all prefs to the default value including prefs set via user.js and prefs that are no longer supported in the current Firefox release.

• http://kb.mozillazine.org/Preferences_not_saved
• http://kb.mozillazine.org/Resetting_preferences