Comment 9 for bug 1226952

This isn't really a bug (or certainly not something we're going to change in Ubuntu). Firefox is single instance, so when you try to run new Firefox instance when there is one already running, the entire command line is delegated to the already open instance.

If you want to use "-Profilemanager" when you already have Firefox open, then you need to also use "-no-remote" to ensure that a new Firefox instance is opened (which is documented). We even state this in the manpage that we add to Firefox.

"-u" is not documented because it's an internal implementation detail. It "works" in your case because you are using it incorrectly which results in the remoting of the command line failing.