Firefox 52 will use the profile that is marked as Default=1 in profiles.ini or the profile you specify with the "-P <profile-name>" command line switch in a desktop shortcut.

• https://developer.mozilla.org/en-US/Command_Line_Options

Firefox 67 and later can use a dedicated profile and lock this profile against other Firefox installations.

• https://support.mozilla.org/en-US/kb/dedicated-profiles-firefox-installation

Firefox 52 ESR and 60 ESR do not support this profile lock (you need Firefox 67 or later). I don't know whether Firefox 69 modifies profiles.ini and changes the Default profile setting in some cases. The most problematic is what happens if you open external links in Firefox (default browser and default profile). If Firefox 52 is the default browser then it uses the Default=1 profile without warning. Firefox 69 is smart enough to find and use its own dedicated profile.

Hello cor-el,

Thanks for replying.

Do you know where "C:\Users\---\AppData\Roaming\Mozilla\Firefox" is set in FF 52 source code?

Or perhaps a Registry tweak?

This isn't specifically coded in Firefox. This is done by calling Windows Shell functions to get known folder IDs with a possible fallback to the Windows registry. See GetWindowsFolder (#101) and GetRegWindowsAppDataFolder (#179)

• https://dxr.mozilla.org/mozilla-esr52/source/xpcom/io/SpecialSystemDirectory.cpp
• https://dxr.mozilla.org/mozilla-esr52/source/xpcom/io/nsDirectoryServiceDefs.h

Vista and later support SHGetKnownFolderPath directly.

• https://docs.microsoft.com/en-us/windows/win32/api/shlobj_core/nf-shlobj_core-shgetknownfolderpath
• https://docs.microsoft.com/en-us/windows/win32/shell/knownfolderid

cor-el,

Thank you very much for the detailed and helpful reply. I do appreciate that.

I was hoping it would be at the JS level (I've never built FF).