Update for Firefox 136: The setting to choose between completely closing the sidebar (old style) and showing vertical button bar (new style) moved. See: Use the sidebar to access tools and vertical tabs.

Hi, Firefox's sidebar is changing -- it has a new design that expands from and collapses back to a vertical button bar (launcher) instead of hiding completely. If you want to hide it completely, that's a two-step process:

(1) Change the setting for what the Sidebars toolbar button does, and (2) Click the Sidebars toolbar button to hide the sidebar

Sidebar Button Setting

Click the gear wheel at the bottom of the vertical button bar, then change the behavior of the button from expand/collapse to show/hide. I'm attaching a picture for reference:

After you make that change, you can use the X to close the "Customize sidebar" panel.

Click the Sidebars button

This button should be near the back/forward/reload buttons. If yours is missing, you can add it using the Customize feature (Customize Firefox controls, buttons and toolbars). I'm attaching a picture for reference:

Success?

Thank you for this. If i"m interpreting what I see on my screen, the sidebar content is a choice among bookmarks, history, synced tabs, and AI chatbot. Why isn't it also used for my open tabs and eliminating the standard horizontal tabs. Or is that available by some other means? Stacking the tabs vertically means more are visible and easier to read while sacrificing little web page space on a wide screen.

I didn't show the whole panel for Customize sidebar in my screenshot. If you scroll down, I think that is where you find the Vertical tabs option.