Try to find an option to disable that.

Opening the sidebar is a global setting, so if you open the sidebar in the main window then the sidebar opens in additional windows you open. There is no way to prevent this otherwise then keeping the sidebar closed in one window and open new window from the window that has the sidebar closed.

Hi Deltadervish, if the sidebar is open in any window, it will also be open in a new window. Can you close the sidebar in all the other windows as a workaround?

Sometimes the settings file that stores window dimensions and sidebar status 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 Enter

In the first table on the page, on the Profile Folder row, click the "Open Folder" button. This should launch a new window listing various files and folders in Windows Explorer.

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

• "3-bar" menu button > Exit
• (menu bar) File > Exit

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 again. Do windows display normally again?

That mini-reset will hide the menu bar and 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 Bookmarks Toolbar, Menu Bar, or other bars, click it on the list.

If I close the sidebar in one window (this window was opened from a window that has the sidebar) and reopen a new window from this window then the new window doesn't open with the sidebar.