User Agent: Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:65.0) Gecko/20100101 Firefox/65.0
Steps to reproduce:
Start Nightly, get a message that profiles are now separated.
Attempt to start a release Firefox (/usr/bin/firefox), get a new window in the Nightly instance.
/usr/bin/firefox --new-instance, get a new instance of release Firefox.
Thus far, all is good.
Close Nightly and attempt to start a new instance of it while Release is still running:
A message box pops up with the good old message: "Firefox is already running, but is not responding. To open a new window, you must first close the existing Firefox process, or restart your system."
A new Nightly instance should launch.