That is not possible. If you hide this button then you won't be able to access/manage extensions in the future. *https://support.mozilla.org/en-US/kb/extensions-button Firefox 109 comes with the new Extensions button as the new way to manage your extensions. The Extensions button (puzzle piece icon) drop-down serves as overflow area for extensions you install, the classic chevron (>>) overflow area still exists but is only used for built-in toolbar buttons as found in the Customize palette. You can click the gear icon next to an extension or use the right-click context menu to pin the extension to the toolbar (Pin to Toolbar). To (re)move the extension from the toolbar back to the Extensions button drop-down, you can use "Unpin from Toolbar" in the right-click context menu. If you do not have extensions installed then this button takes you to about:addons (Tools -> Add-ons -> Extensions). The regular overflow area (>>) can only be used for built-in Firefox toolbar buttons as shown in the Customize palette and can no longer be used for extensions. For buttons provided by extensions, the Extensions button drop-down serves as overflow area ("Pin to Toolbar" and "Unpin from Toolbar").