Comment 1 for bug 308605

> Steps to Reproduce:
> 1. Start firefox with an existing profile "default"
> 2. Start firefox with an existing profile "second" using 'firefox -P second'
> Actual Results:
> The running firefox instance using the profile "default" opens another window

> 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.

Design/Behaviour of "Remote mode" and "NoRemote mode" is same as current since initial of Mozilla family. Simply, command line parameter for the mode is changed to one used by MS Win version from one used by Mozilla on Linux(Called "Mozilla Application Suite" after release of Fx/Tb).
(Before change on Linux)
  "Remote mode" : Started with -remote
  "NoRemote mode" : Started without -remote
(After change on Linux.)
  "Remote mode" : Started without -no-remote
  "NoRemote mode" : Started with -no-remote
And, "-P prof_name" has meaning only when (A) start of first instance of Firefox of "Remote mode", (B) start of new Firefox instance of "No-Remote mode".
See Bug 308076 Comment #19 for brief history of "-remote" switch, MOZ_NO_REMOTE=1 environment variable, and "-no-remote" switch.
See also Bug 459535(same report/claim of same behaviour of Seamonkey trunk on Linux).
Do you really think that enhancement(==Design change around "Remote/NoRemote mode") has to be done?
Please note that usual/average users never use multiple profiles. Further, many of usual/average users don't know about existence of profile manager, because Firefox developers(Mozilla Company) eager to hide existence of profile manager.
See following Bug.
> Bug 214675 Remove Profile Manager UI

> "Firefox is already running, but is not responding. [..]"

Which version of Firefox is the "unresponsive" Firefox?
How the "unresponsive" Firefox is started before the "unresponsive" problem?
Similar problem on Linux to Bug 427118 on MS Win(DUP of Bug 395891 on MS Win)?
Anyway, the "unresponsive" problem is different from your enhancement request, although the enhancement will be possibly a solution of the "unresponsive" problem. I recommend you to open separate bug for the "unresponsive" problem.