I did that before and now and have no problem opening two separate Browser Windows. I think you have something cause the problem.

2 or more instances, or windows?

For more than one instance, you need the command line option; -no-remote

Here is an example (on one line only);

C:\PROGRA~1\MOZILL~1\firefox.exe -no-remote -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.
 
 -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.

edited out: The -new-instance is a old switch and not related to Firefox instances and does not work on Windows.

Thanks for your replies.

Never had the problem before in opening two Firefox browsers.

I certainly am not going to enter into command line options - looks like asking for more problems.

Any ideas what has changed to stop me opening more than one browser?

Well - problem solved itself - it's back working again.

I sometimes wonder who or what owns this computer?

Thanks for your replies. Regards

Without more information on the problem . . . .

After a few days, if the problem is gone, Please flag your last post as Solved Problem so others will know.

-UILocale <locale> no longer works in current Firefox releases (55+).