Comment 22 for bug 308605

(In reply to comment #0)
> Starting another firefox instance can be done by
> firefox -P second -no-remote
> but there is no way to e.g. open a firefox window with a specified profile and
> a given url from e.g. command line or from other programs that allow to specify
> the browser command.
> Running that very same command again if there is already a firefox instance
> using the profile "second" shows a dialog with the (only partially correct)
> message "Firefox is already running, but is not responding. [..]"

I think above your case is same as Bug 491295. But your result is different from one reported by Bug 491295. Simly because different build? (6 months, so big difference)
Frank-Ralph Reiser, can you reproduce "Firefox is already running, but is not responding. [..]" with newest Fx trunk nightly?