I may have misread your post, but it sounds like just closing the window should solve your problem.

If you want to close all Firefox windows;

Press the <Alt> or <F10> key. Select File, then Exit.

If you want, there are add-ons that can put an icon on your tool bar that would close Firefox.

Go to the Mozilla Add-ons Web Page {web link} (There’s a lot of good stuff here) and search for what you want.

Quit Firefox {web link} Quit Firefox provides statusbar and Toolbar buttons to quickly shut down Firefox. It is especially useful for laptop users.

If you want to close Firefox then you can find the Firefox close (Exit/Quit) button () in the "3-bar" Firefox menu button drop-down list.

You can also still do this via the File menu.

• Tap the Alt key or press F10 to show the Menu Bar

• https://support.mozilla.org/kb/learn-more-about-the-design-of-new-firefox
• https://support.mozilla.org/kb/common-questions-after-updating-to-new-firefox