You can toggle browser.preferences.inContent to false on the about:config page.

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

Thank you so much!!!

This tip worked great when I used it a few months ago. I just upgraded to Firefox 42.0, and now Options opens in a tab again instead of a new window. I checked the about:config page and browser.preferences.inContent is still set to false. Is there any way to change this behavior in version 42.0?

The browser.preferences.inContent is no longer supported in Firefox 42 and later (Bug 1140495), so it is not possible to open a standalone window by setting this pref to false.

You can possibly create a button on the Bookmarks Toolbar to the about:preferences page and hold the Shift key and left-click to open the page in a new window.

You can look at the Classic Theme Restorer extension.

• General UI (1): Open options page (about:preferences) in a window

If that isn't working or the choice is missing then you might be using a theme that isn't supported. You can check that by switching to the default theme.

Thanks for your help. The Classic Theme Restorer extension did the trick.

You're welcome.

I've changed to solution to my above reply to avoid confusion.