As to the add-on, Here are your choices; 1) Contact its support and ask if they will update 2) Find a replacement 3) Downgrade to a prior Firefox 4) Do without

You can use the Windows command line or batch file to start Firefox with the profile you want.

Example; (One Line) C:\PROGRA~1\MOZILL~1\firefox.exe -no-remote -new-instance -new-window -P Firefox_v_57

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.

What ever happened to the command firefox.exe -p from the run window?

That command line switch should still work. You may have to use the full path in case Windows doesn't find the Firefox.exe file.

• https://developer.mozilla.org/Command_Line_Options
• https://developer.mozilla.org/Mozilla/Multiple_Firefox_Profiles

supercain3 said

What ever happened to the command firefox.exe -p from the run window?

The ProfileManager still works in Firefox 57.0 and later.

A key thing people sometimes miss is making sure Firefox is completely closed/exited first before trying to start Firefox with the ProfileManager. Otherwise a new Firefox window will open instead.

Look at the end of my example; -P Firefox_v_57 https://support.mozilla.org/en-US/questions/1199529#answer-1064022

Thank you for your answers. The profile can be switched indeed only when all instances of Firefox are closed but that means then its not possible to have many instances with different profiles open at the same time?

supercain3 said

...but that means then its not possible to have many instances with different profiles open at the same time?

It is, you just need to use the -no-remote switch to start another instance with another Profile. You can set a shortcut to start with a specific Profile or select in ProfileManager. Make sure you do not use this with the default Profile.

http://kb.mozillazine.org/Opening_a_new_instance_of_Firefox_with_another_profile

You can only use one Profile for each instance. It has been this way for a very long time since Firefox 1.5 released way back November 30, 2005. In the Releases before up to including the 1.0 Release (that some people incorrectly think Firefox started at) you could start another Firefox instance with a specific Profile or selecting one in ProfileManager. Firefox 1.5 used MOZ_NO_REMOTE and 2.0 to present used -no-remote

I get it now. Thank you very much everyone for your help.

Actually Firefox sets MOZ_NO_REMOTE=1 internally when you use the -no-remote command line switch.

• https://dxr.mozilla.org/mozilla-central/source/toolkit/xre/nsAppRunner.cpp#3600