Comment 9 for bug 1278062

@Chris Coulson. Just had a look into /var/log... it seems there is a failure earlier, and not just for firefox.

gnomeplayer reported a crash...
ERROR: apport (pid 16477) Mon Feb 24 10:08:16 2014: called for pid 16447, signal 5, core limit 0
ERROR: apport (pid 16477) Mon Feb 24 10:08:16 2014: executable: /usr/bin/gnome-mplayer (command line "/usr/bin/gnome-mplayer --window=54102218 --controlid=25263 --width=1600 --height=674 --autostart=1 --showcontrols=1")
ERROR: apport (pid 16477) Mon Feb 24 10:08:16 2014: gdbus call error: Error: GDBus.Error:org.freedesktop.DBus.Error.ServiceUnknown: The name org.gnome.SessionManager was not provided by any .service files

ERROR: apport (pid 16477) Mon Feb 24 10:08:16 2014: debug: session gdbus call:
ERROR: apport (pid 16477) Mon Feb 24 10:08:22 2014: wrote report /var/crash/_usr_bin_gnome-mplayer.1000.crash
ERROR: apport (pid 17555) Mon Feb 24 11:18:04 2014: called for pid 4535, signal 11, core limit 0

Now when firefox then crashed out.....

ERROR: apport (pid 17555) Mon Feb 24 11:18:04 2014: executable: /usr/lib/firefox/firefox (command line "/usr/lib/firefox/firefox")
ERROR: apport (pid 17555) Mon Feb 24 11:18:04 2014: gdbus call error: Error: GDBus.Error:org.freedesktop.DBus.Error.ServiceUnknown: The name org.gnome.SessionManager was not provided by any .service files

ERROR: apport (pid 17555) Mon Feb 24 11:18:04 2014: debug: session gdbus call:
ERROR: apport (pid 17555) Mon Feb 24 11:19:18 2014: wrote report /var/crash/_usr_lib_firefox_firefox.1000.crash

Hope this is of help... I've taken a backup of /var/log in case you need it in its entireity (I did some re-boots, tried re-installing etc).