Comment 1 for bug 281958

Yup, same here. I tried purging firefox and flashplugin-nonfree, as well as deleting my mozilla profile.
Here's the contents of /usr/lib/firefox/plugins/:
flashplugin-alternative.so mplayerplug-in-rm.xpt
mplayerplug-in-dvx.so mplayerplug-in.so
mplayerplug-in-dvx.xpt mplayerplug-in-wmp.so
mplayerplug-in-qt.so mplayerplug-in-wmp.xpt
mplayerplug-in-qt.xpt mplayerplug-in.xpt
mplayerplug-in-rm.so

BTW, flash does NOT appear in firefox's about:plugins