I used to be able to use a pulldown menu (I think it was in "view") to select another window that was open in Firefox; doesn't seem to show that now.

Is there a tool or an icon I can add that will allow me to easily switch to an already open window?

Don't know of anything in Firefox to direct you to another already open Firefox Window.
Next to the close button at the end of the tabs strip, or as the last button on the tabs stripe you have the "List all Tabs" drop-down, which s for viewing tabs that exist in the same window.
The view menu has sidebars, and you can bring up a sidebar but that is not another window.

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