Oops, I misread the question.

Maybe this can be of help.

Note that the OP asked for the bookmarks sidebar.

When do you no longer see the bookmarks sidebar? Is that when you close and restart Firefox?

Try to rename/remove the xulstore.json file in the Firefox profile folder with Firefox closed.

You can use the button on the "Help -> Troubleshooting Information" (about:support) page to go to the current Firefox profile folder or use the about:profiles page.

• Help -> Troubleshooting Information -> Profile Directory:
  Windows: Show Folder; Linux: Open Directory; Mac: Show in Finder
• http://kb.mozillazine.org/Profile_folder_-_Firefox

That is an interesting add-on: Sidebar Auto Show/Hide. I don't think it's going to be possible to create a new extension that works that same way -- showing the sidebar when you hover in that area and then closing it automatically.

Until a solution is discovered, can your fingers get used to using Ctrl+b? That should open and shut the Bookmarks Sidebar as long as the page doesn't intercept it. (For example, an email compose page may intercept it and turn on "bold" when you press that shortcut.)

Renaming xulstore didn't work. None of the addons mentioned work. It's a sidebar I'm after, not a toolbar. I've got a lot of bookmarks. :)

Can you clarify in what way the bookmarks sidebar isn't working and gets hidden?

The sidebar doesn't display by default. It's available via Menu bar > View > Sidebar > Bookmarks. (There may be another way to get at it....)

Of course, the Menu Bar doesn't display by default, either. I think I found it only by using Classic Theme Restorer.

"... think I found it only by using Classic Theme Restorer."

Which is a legacy extension that won't work in Firefox 57-plus.

If the Bookmarks Sidebar is open when Firefox is closed properly (File > Exit and other), it should be open the next time Firefox is launched. It would remain open as long as the user doesn't close it; unless an extension is installed that fiddles with that built-in feature.

Note that you can tap the Alt key or press the F10 key to show the hidden Menu Bar temporarily.

@the-edmeister: it does. Some features of it don't seem to work, but most I've enabled do.

<quote>If the Bookmarks Sidebar is open when Firefox is closed properly (File > Exit and other), it should be open the next time Firefox is launched. It would remain open as long as the user doesn't close it; unless an extension is installed that fiddles with that built-in feature.</quote>

In my experience it has been years since the bookmarks sidebar has been a default display feature.

Exit and other? From File in the Menu bar? Not here. So not there? Closed properly? Like, not shut down by OS shutdown? I never do that.

....sigh..... This is a desktop version. Why it's set up like a mobile version defies my comprehension. Yet it retains keyboard shortcuts (most of which I've not used).....

Hi A_Culture_Mind, to turn different bars on and off, you can use one of these methods to show the toolbar list and select the desired bars there:

• right-click (on Mac Ctrl+click) a blank spot on the tab bar (or the "+" button)
• (menu bar) View > Toolbars
  On Windows, tap the Alt key to activate the menu bar temporarily.
• in Customize mode > Show/Hide Toolbars (see: Customize Firefox controls, buttons and toolbars)

To switch the visibility of the Menu Bar, Bookmarks Toolbar, or other bars, click it on the list.