They must be installed on two different folder to run separately. If you didn't do this it will not work.

Are you talking about running Firefox as separate windows/profiles?

I sometimes run two different versions of Firefox. C:\PROGRA~1\MOZILL~1\firefox.exe -no-remote -new-instance -new-window -P Firefox_v_57

https://developer.mozilla.org/en-US/docs/Mozilla/Command_Line_Options

Usage: C:\PROGRA~1\MOZILL~1\firefox.exe [ options ... ] [URL]
      where options include:
 
 -h or -help        Print this message.
 -v or -version     Print Firefox version.
 -P <profile>       Start with <profile>.
 -migration         Start with migration wizard.
 -ProfileManager    Start with ProfileManager.
 -no-remote         Do not accept or send remote commands; implies -new-instance.
 -new-instance      Open new instance, not a new window in running instance.
 -UILocale <locale> Start with <locale> resources as UI Locale.
 -safe-mode         Disables extensions and themes for this session.
 -console           Start Firefox with a debugging console.
 -jsconsole         Open the Error console.
 -browser           Open a browser window.
 -new-window  <url> Open <url> in a new window.
 -new-tab     <url> Open <url> in a new tab.
 -preferences       Open Options dialog.
 -search     <term> Search <term> with your default search engine.
 -setDefaultBrowser Set this app as the default browser.

FredMcD said

Are you talking about running Firefox as separate windows/profiles? I sometimes run two different versions of Firefox. C:\PROGRA~1\MOZILL~1\firefox.exe -no-remote -new-instance -new-window -P Firefox_v_57 https://developer.mozilla.org/en-US/docs/Mozilla/Command_Line_Options

Why are you using the -new-instance when it does nothing on Windows as mentioned on same page. You are also not using the -new-window switch properly as it is for opening a url in a new window.

-new-instance Note: Not available for Windows, see bug 855899.

If you use the -no-remote command line switch then you should be able to run multiple Firefox versions. If you want to be able to open external links in Firefox then you should start the default browser without the -no-remote switch. The -no-remote switch hides this instance from the OS and you can't open external in such an instance.

See also:

• https://developer.mozilla.org/en-US/Mozilla/Multiple_Firefox_Profiles

Thanks for all the help and sorry for making my original post so cryptic. I have FireFox as an icon on the Win 10 taskbar. When I click the icon FireFox opens. Previously when I right-clicked the icon I could open a new window and FireFox would open in it. This process ceased to work a few days ago and hence my question to the forum. To-day, however, the process works again. I can now open a second FireFox window as I use to by right-clicking the task-bar icon. I can offer no explanation!

Brian