Hi, from v38 the Options (cog wheel) settings now open in a tab, rather than a small window. You can close it as you would any other tab, by clicking on the 'x' - Close Tab, or right click > Close Tab.

If you prefer, you can go back to the old (window) version by: Type about:config into the address bar, press Enter, accept the warning, scroll down to browser.preferences.inContent and double click to change 'Value' from True to False taking care not to alter anything else.

If your question is resolved by this or another answer, please take a minute to let us know. Thank you.

Thank you so much. That was a perfect solution. I just wish I knew how it was changed in the first place.

Firefox’s Redesigned Preferences Feel More like the Web:

• https://blog.mozilla.org/ux/2014/05/firefoxs-redesigned-preferences-feel-more-like-the-web/

Last time I used the Config step, I lost all my bookmarks. I did not change anything else, so I will not do this method again.

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

You can look at the Classic Theme Restorer extension.

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

• https://addons.mozilla.org/firefox/addon/classicthemerestorer/