If local Firefox is running, I can run Firefox Portable only with AllowMultipleInstance, but after that I can't open new window

My application needs to execute some URL. I need to open this URL in Firefox Portable. If there is a already installed and running other Firefox, than my Firefox Portable, FP will not execute (There is well know message about another running instance).
If I set AllowMultipleInstances to true, I can execute Firefox Portable window with desired URL. Problem is, if I make another call from my application to execute another URL, Firefox Portable will fail to load another window of the same instance that already running. It's probably trying to run another (seconf) instance of process on same profile.

Is there any workaround to my situation?
Thanks for any help.

(If you want to questions my reasons: I need to show URL content in customized Firefox portable, because I don't want to rely on external user's browser. Instead I want to have always same customized window without toolbars and other navigation buttons).