Ok, if Thunderbird is running and you try and start Firebird, the MozillaFirebird script thinks that Firebird is already running and tries to open a new window/tab. It can't and so produces "Unable to send command to running Mozilla Firebird instance". I think this is actually a problem with mozilla-xremote-client. Anyway, I bodged around this by not checking for the return value and just loading a new Firebird if mozilla-xremote-client failed. There are other problems though which may be symptoms of the bodge...I'm still working on this and will report on any findings...
*** This bug has been marked as a duplicate of 40179 ***