New profile manager: allow for command line -P to work with the new profile system.

[Berstarke]

Right now having two different profile systems is a hassle. I assume the old system will be phased out eventually, but in the meanwhile I would like to have command line options so I can set dedicated shortcuts for firefox to always open with specific profiles. Either updating -P to work with the new system, or as a separate option is fine.

[kelimuttu]

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