On MDN, it states here that the element ID for the Bookmarks sidebar is reported by

• CustomizableUI.AREA_BOOKMARKS

Can someone please tell me how I can use that to force Firefox to hide the Bookmarks sidebar? I don't want a "toggle" function because, I need to ignore whichever state it might be in, at the time of the JavaScript call from the loaded HTML page, and force a "hidden", "collapsed" or "not-displayed" mode.

I tried the following as inline scripting, but it did not have the desired effect, likely because I don't have the proper references to tell the browser properly.

<script>
    var sidebar = browser.getElementById("CustomizableUI.AREA_BOOKMARKS") ;
    sidebar.hide() ;
</script>

I've also tried the following without success:

<script>
    browser.action.closePanel ;
    browser.action.closeSidebar ;
</script>

Finally, even the specific directive given here does not work:

<script>
    browser.sidebarAction.close() ;
</script>

Help, please!

To clarify, this action must work in the new window opened at command line,

firefox -new-window ./myPage.html &

even while a pre-existing browser session is operating with the Bookmarks sidebar open (and staying open).