This is a guess, but I believe the most recent update of linux-c6-gtk2 (r432937) has broken flashplayer. Flashplayer does not start after some package updates and I've been trying to pin down what broke it through dependencies. The reason for this guess is that the error message comes from the /compat/linux/lib/libgtk-x11-2.0.so.0 sharedlib used by nspluginwrapper. The message is the following: `npviewer.bin:(pid): Gtk-WARNING **: cannot open display: :0` whenever firefox tries to startup the flash plug-in.
Edit: I just realized that r432937 touch quite a few ports, so the target is likely a lot bigger... I strongly believe it was something from this revision, however.
Looks like this was fixed recently.