I have the same problem here, and I've noticed that even after I kill firefox I have a bunch of plugin-container processes left around:

# Firefox main process not running:
$ ps ax|grep firefox|grep -v plugin
 6013 pts/11   S+     0:00 grep --color=auto firefox

# but...

$ ps ax|grep firefox
 3931 ?        Sl     0:00 /usr/lib/firefox-7.0/plugin-container /var/lib/flashplugin-installer/npwrapper.libflashplayer.so -greomni /usr/lib/firefox-7.0/omni.jar 2770 true plugin
 4001 ?        Sl     0:00 /usr/lib/firefox-7.0/plugin-container /var/lib/flashplugin-installer/npwrapper.libflashplayer.so -greomni /usr/lib/firefox-7.0/omni.jar 2770 true plugin
 4049 ?        Sl     0:00 /usr/lib/firefox-7.0/plugin-container /var/lib/flashplugin-installer/npwrapper.libflashplayer.so -greomni /usr/lib/firefox-7.0/omni.jar 2770 true plugin
 4248 ?        Sl     0:00 /usr/lib/firefox-7.0/plugin-container /var/lib/flashplugin-installer/npwrapper.libflashplayer.so -greomni /usr/lib/firefox-7.0/omni.jar 2770 true plugin
 4296 ?        Sl     0:00 /usr/lib/firefox-7.0/plugin-container /var/lib/flashplugin-installer/npwrapper.libflashplayer.so -greomni /usr/lib/firefox-7.0/omni.jar 2770 true plugin
 5225 ?        Sl     0:00 /usr/lib/firefox-7.0/plugin-container /var/lib/flashplugin-installer/npwrapper.libflashplayer.so -greomni /usr/lib/firefox-7.0/omni.jar 2770 true plugin
 5247 ?        Sl     0:00 /usr/lib/firefox-7.0/plugin-container /var/lib/flashplugin-installer/npwrapper.libflashplayer.so -greomni /usr/lib/firefox-7.0/omni.jar 2770 true plugin
 5270 ?        Sl     0:00 /usr/lib/firefox-7.0/plugin-container /var/lib/flashplugin-installer/npwrapper.libflashplayer.so -greomni /usr/lib/firefox-7.0/omni.jar 2770 true plugin
 5289 ?        Sl     0:00 /usr/lib/firefox-7.0/plugin-container /var/lib/flashplugin-installer/npwrapper.libflashplayer.so -greomni /usr/lib/firefox-7.0/omni.jar 2770 true plugin
 5433 ?        Sl     0:00 /usr/lib/firefox-7.0/plugin-container /var/lib/flashplugin-installer/npwrapper.libflashplayer.so -greomni /usr/lib/firefox-7.0/omni.jar 2770 true plugin
 5480 ?        Sl     0:00 /usr/lib/firefox-7.0/plugin-container /var/lib/flashplugin-installer/npwrapper.libflashplayer.so -greomni /usr/lib/firefox-7.0/omni.jar 2770 true plugin
 5586 ?        Sl     0:00 /usr/lib/firefox-7.0/plugin-container /var/lib/flashplugin-installer/npwrapper.libflashplayer.so -greomni /usr/lib/firefox-7.0/omni.jar 2770 true plugin
 5751 ?        Sl     0:00 /usr/lib/firefox-7.0/plugin-container /var/lib/flashplugin-installer/npwrapper.libflashplayer.so -greomni /usr/lib/firefox-7.0/omni.jar 2770 true plugin
 5852 ?        Sl     0:00 /usr/lib/firefox-7.0/plugin-container /var/lib/flashplugin-installer/npwrapper.libflashplayer.so -greomni /usr/lib/firefox-7.0/omni.jar 2770 true plugin
 5879 ?        Sl     0:00 /usr/lib/firefox-7.0/plugin-container /var/lib/flashplugin-installer/npwrapper.libflashplayer.so -greomni /usr/lib/firefox-7.0/omni.jar 2770 true plugin
 5906 ?        Sl     0:00 /usr/lib/firefox-7.0/plugin-container /var/lib/flashplugin-installer/npwrapper.libflashplayer.so -greomni /usr/lib/firefox-7.0/omni.jar 2770 true plugin
 5976 ?        Sl     0:00 /usr/lib/firefox-7.0/plugin-container /var/lib/flashplugin-installer/npwrapper.libflashplayer.so -greomni /usr/lib/firefox-7.0/omni.jar 5928 true plugin
 6021 pts/11   S+     0:00 grep --color=auto firefox

this is with firefox 7 on up-to-date Oneiric:

$ dpkg-query -W firefox
firefox	7.0~b1+build1+nobinonly-0ubuntu1