Start firefox in safe mode:
This will start a firefox session with all add-ons disabled (temporarily). Just keep one single tab open (say, www.google.com
). Then see if it makes any difference.
If your screensaver kicks-off, then the culprit is some add-on. Identify the issue by disabling them one by one and retrying.
If your screensaver stil refuses to kick-of, check $HOME/.xsession-errors (I can't remember the filename exactly).
You can also kick-off firefox with -console option for debugging info. Also check the /var/log/syslog & /var/log/messages + any gnome-screensaver specific logs (if any).
You may also want to find out if you can start gnome-screensaver with some debug flag on.
Can't think of anything else