First, could you try the menu bar:

View > Sidebar > Bookmarks

That also may show the keyboard shortcut, Command+b

Does that work or is that no longer working?

If you can't see a sidebar when switching it on and off from the menu, its width might be too narrow.

Sometimes the settings file that stores dimensions of windows, dialogs, and toolbar controls becomes corrupted with nonsensical values. You can rename the file and have Firefox rebuild it. Here's how:

Open your current Firefox settings (AKA Firefox profile) folder using either

• "3-bar" menu button > "?" Help > Troubleshooting Information
• (menu bar) Help > Troubleshooting Information
• type or paste about:support in the address bar and press Return

In the first table on the page, on the Profile Folder row, click the "Show in Finder" button. If Finder highlights an icon with a semi-random name like a1b2c3d4.default, double-click it to display the contents of that profile folder.

Leaving that window open, switch back to Firefox and Quit, either:

• "3-bar" menu button > Quit
• (menu bar) Firefox > Quit

Pause while Firefox finishes its cleanup, then rename xulstore.json to something like xulstore.old. If you see a file named localstore.rdf, rename that one to localstore.old.

Start Firefox back up. Does the sidebar display normally again?

That mini-reset will hide the bookmarks toolbar. 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 > Toolbars button (see: Customize Firefox controls, buttons and toolbars)

To turn on the Menu Bar, or other bars, click it on the list.