Press “Ctrl,” “Shift” and “B” to bring it back (or “Command,” “Shift” and “B” on Mac). If the problem keeps coming back, you can click the three dots to go to the menu, choose “Settings” and then “Appearance.” Make sure “Show the bookmarks bar” is set to “On," and then exit settings.

The shortcut to toggle the sidebar on Mac is "Command+B" (Command+Shift+B toggles the Bookmarks Toolbar). You may have an extension that causes this.

Start Firefox in Troubleshoot Mode to check if one of the extensions ("3-bar" menu button or Tools -> Add-ons -> Extensions) or if hardware acceleration or if userChrome.css/userContent.css is causing the problem.

• switch to the DEFAULT theme: "3-bar" menu button or Tools -> Add-ons -> Themes
• do NOT click the "Refresh Firefox" button on the Troubleshoot Mode start window

• https://support.mozilla.org/en-US/kb/diagnose-firefox-issues-using-troubleshoot-mode
• https://support.mozilla.org/en-US/kb/troubleshoot-extensions-themes-to-fix-problems