CTRL+o (letter O) will close all but the selected tab on my system. On the right-click on tabs drop down, see the underlined "o" in "Close other tabs"?

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The documentation states that CTRL-o is for the Open File dialogue, and it does bring up that dialogue for me. Not helpful.

And, yes, I do see the "o" shortcut for "Close Other Tabs" when that menu is in view, and it does work when that menu is in view. But I still have no keyboard method for getting the menu itself!

You are correct. Sorry about that, my old brain does not always work the way it should.

Look at this Extension --> https://addons.mozilla.org/en-US/firefox/addon/close-other-tabs-1/

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I clicked "Solved It", since the extension does work. However, this does not seem to me to be a very satisfactory solution. It should not require an addon to solve this problem. What is the point of having a keyboard accelerator key on the tabs context menu (the "o") if you cannot get the menu itself with just a keyboard?

See if this post by the-edmeister in the Mozillazine forum works for you --> http://forums.mozillazine.org/viewtopic.php?p=8535065#p8535065

Also see --> https://en.wikipedia.org/wiki/Menu_key

On my system this works when on the selected tab to save: CTRL+K, Tab, {ContextMenu key} or SHIFT+F10, o (letter o on keyboard)

I have never used that key before. Thanks to you, DrHow, I learned something too.

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I had tried to use Menu_key myself; but I had not figured out how to get the focus on a tab (not its page) to start with. The CTRL-k Tab trick does put the focus on the tab the page for which is being displayed. In any case, you must first display the page for the tab you want to keep. IMO, the extension remains the best solution. The forum thread just confirms my notion that lack of this functionality is an oversight in the design which deserves to be addressed in the browser itself.

"...you must first display the page for the tab you want to keep."

Isn't that a given when trying to save (not close) just one tab of many? Otherwise, what tab are you trying to save? After focusing on the Tab Bar (Ctrl_K, Tab), you can use the right and left keyboard arrows to choose any other already open tab, is that is what concerns you. Also note that CTRL+L will focus on the Location/URL bar and will require 2 tabs following to get to the Tab Bar (I have not tested, but that may be necessary for anyone who has removed the Search Bar from the UI and need to get focus on the Tab Bar).

You can send a short comment to the developers concerning making the tab context menu items more easily accessible by using Help > Submit Feedback. Due to the number of shortcuts already available in Firefox and the need for Extensions that already use many keyboard shortcut key combinations, it might be difficult for the developers to find a keyboard combination that does not conflict with an already-used extension shortcut, that might be utilized by a few users out of millions. My experience on this forum indicates that the large majority of users are not concerned about and/or are not using keyboard shortcuts.