Comment 1 for bug 1266015

It depends on the flash clip you are trying to play.
See
http://wiki.gnashdev.org/FAQ#What_should_gnash_play.3F
http://bugs.debian.org/724732#10

--
BTW real conflict is between browser-plugin-lightspark and flashplayer-installer (adobe one):

/etc/alternatives/flash-mozilla.so -> /usr/lib/lightspark/liblightsparkplugin.so
/etc/alternatives/mozilla-flashplugin -> /usr/lib/flashplugin-installer/libflashplayer.so

Ubuntu maintainers renamed gnash alternative from flash-mozilla.so to mozilla-flashplugin, but they didn't rename lightspark one, so if you install both lightspark and adobe ones, browser sees both.
about:plugins page confirms that.
User should be able to switch among them with:
$ sudo update-alternatives --config mozilla-flashplugin # on Ubuntu, whenever lightspark alternative is renamed as well
$ sudo update-alternatives --config flash-mozilla.so # on Debian

Fix
# update-alternatives --remove "flash-mozilla.so" /usr/lib/lightspark/liblightsparkplugin.so
# update-alternatives --install "/usr/lib/mozilla/plugins/flashplugin-alternative.so" "mozilla-flashplugin" /usr/lib/lightspark/liblightsparkplugin.so 0