When you say a new instance, do you mean a completely separate session of Firefox which doesn't share any history with the live one, or a separate new window instead of a new tab? I'll give the steps for the simpler one:

New Window

When you right-click a shortcut and click Properties, Firefox should open the Shortcut tab and highlight the Target. This will show you how Firefox is starting now. For example (assuming yours is installed in the same folder as mine):

"C:\Program Files\Mozilla Firefox\firefox.exe"

To add a specific web address to the target, it would look like this (make sure to keep the spaces before and after the -new-window command-line switch):

"C:\Program Files\Mozilla Firefox\firefox.exe" -new-window https://www.mozilla.org/

You can test your new target command line using the Windows 10 search box or the Run dialog (Windows key + R).

Once you know exactly what the shortcut command line needs to be, you can create a new shortcut by right-clicking a blank area of the desktop, New > Shortcut and follow the steps in the wizard.

Success?

Many thanks for this rapid response.

Using your straighforward recipe I got a new stripped-down window with one tab containing the site I want. However, when I shut down the other Firefox window with its numerous tabs and then started it again it restarted with the stripped-down window with one tab, i.e. all the history was gone (which I didn't want).

Maybe I want your less-simple version. Is that what I called a "new instance" ?

No, this could still happen, so this is essential to know:

If you accidentally close a window and realize it wasn't the one you wanted to close, or if you restore a session and a windows is missing, immediately try:

• Ctrl+Shift+N
• menu button > History > Recently Closed Windows
• (menu bar) History > Recently Closed Windows

To shut down Firefox without accidentally setting any windows to a closed status, close Firefox this way:

• menu button > Exit
• (menu bar) File > Exit

Got it. Worked perfectly (of course). Copy-pasted into my Firefox/IT Tips bag. This rapid help much appreciated.

P.S. Your last posting has been flagged as "Chosen Solution". This is a bit confusing for a later reader because both your postings led stepwise to the solution and so are "Chosen Solutions" which need to be read one after the other.