Set Default Startup Profile for Firefox

To show select profile window on Firefox starup use -P -no-remote
e.g."C:\Program Files (x86)\Mozilla Firefox\firefox.exe" -P -no-remote.

When you start Firefox as normal, It select profile which is selected last.
In normal start up to set default profile modify 
C:\Users\UserName\AppData\Roaming\Mozilla\Firefox\profiles.ini

Add Default=1 which profile need to set default
remove from other profile if found.

save file and make file as read-only.
Read-only file also not allow to create, rename, delete profile.