I tested several plugin versions (64bit and 32bit) with zynga's TH Poker app for facebook, this seems to be a though tesatcase
the only one that get's it completely right is the newest 32bit beta version.
It's a little hard to get it running (because of the required 32bit libraries), but it gives you the best possible compatibility so far.
You will need (of course) the ia32-libs, ia32-libs-gtk, lib32asound2 and lib32asound2-plugins. Additionally, install ia32-libs-libcurl3 from http://www.debian-multimedia.org
Then, you'll need some 32bit libraries from the official firefox release. either install 32bit firefox somewhere in opt or just extract it and make sure to put the following libraries in some directory: libssl3.so libsmime3.so libnss3.so libplds4.so libplc4.so libnspr4.so libsoftokn3.so
Add the directory with these libraries to your ld search path in /etc/ld.so.conf, e.g., i created the file /etc/ld.so.conf.d/local-ia32.conf with just one line:
Run ldconfig, then put the libflashplayer.so from the latest 32bit beta release somewhere and use nspluginwrapper to create a wrapper link in your firefox profile, it should work now.
(if it doesn't, do "ldd /path/to/your/libflashplayer.so" and look for missing libs)