Since I did not receive an answer yet I decided to try it by myself.
It doesn't appear to work, though.
I copied the libflashplayer.so file from my N900 to /home/user/.mozilla/plugins on the N9.
Even though Firefox does not crash when loading a page containing flash content, it would only display a blank page.
ldd on libflashplayer.so gives me the following:
libdl.so.2 => /lib/libdl.so.2 (0x3b1f6000)
libX11.so.6 => /usr/lib/libX11.so.6 (0x3b204000)
libXext.so.6 => /usr/lib/libXext.so.6 (0x3b336000)
libXt.so.6 => not found
libz.so.1 => /usr/lib/libz.so.1 (0x3b34e000)
libcurl.so.4 => /usr/lib/libcurl.so.4 (0x3b372000)
libgstreamer-0.10.so.0 => /usr/lib/libgstreamer-0.10.so.0 (0x3b3c7000)
libxml2.so.2 => /usr/lib/libxml2.so.2 (0x3b472000)
libpulse.so.0 => /usr/lib/libpulse.so.0 (0x3b5ad000)
libosso.so.1 => not found
libplayback-1.so.0 => not found
libhildonfm.so.2 => not found
libgnomevfs-2.so.0 => not found
libgconf-2.so.4 => /usr/lib/libgconf-2.so.4 (0x3b5fb000)
libgthread-2.0.so.0 => /usr/lib/libgthread-2.0.so.0 (0x3b621000)
librt.so.1 => /lib/librt.so.1 (0x3b62d000)
libdbus-glib-1.so.2 => /usr/lib/libdbus-glib-1.so.2 (0x3b63e000)
libdbus-1.so.3 => /usr/lib/libdbus-1.so.3 (0x3b667000)
libpthread.so.0 => /lib/libpthread.so.0 (0x3b6ad000)
libhildon-1.so.0 => not found
libgtk-x11-2.0.so.0 => not found
libgdk-x11-2.0.so.0 => not found
libatk-1.0.so.0 => not found
libgio-2.0.so.0 => /usr/lib/libgio-2.0.so.0 (0x3b6ce000)
libpangoft2-1.0.so.0 => /usr/lib/libpangoft2-1.0.so.0 (0x3b7e6000)
libgdk_pixbuf-2.0.so.0 => not found
libpangocairo-1.0.so.0 => /usr/lib/libpangocairo-1.0.so.0 (0x3b816000)
libcairo.so.2 => /usr/lib/libcairo.so.2 (0x3b828000)
libpango-1.0.so.0 => /usr/lib/libpango-1.0.so.0 (0x3b8cd000)
libfreetype.so.6 => /usr/lib/libfreetype.so.6 (0x3b91a000)
libfontconfig.so.1 => /usr/lib/libfontconfig.so.1 (0x3b9a3000)
libgobject-2.0.so.0 => /usr/lib/libgobject-2.0.so.0 (0x3b9de000)
libgmodule-2.0.so.0 => /usr/lib/libgmodule-2.0.so.0 (0x3ba2e000)
libglib-2.0.so.0 => /lib/libglib-2.0.so.0 (0x3ba39000)
libstdc++.so.6 => /usr/lib/libstdc++.so.6 (0x3bb29000)
libm.so.6 => /lib/libm.so.6 (0x3bc0b000)
libc.so.6 => /lib/libc.so.6 (0x3bc82000)
libgcc_s.so.1 => /lib/libgcc_s.so.1 (0x3bdce000)
/lib/ld-linux.so.3 (0x1f555000)
libxcb.so.1 => /usr/lib/libxcb.so.1 (0x3bde2000)
libcares.so.2 => /usr/lib/libcares.so.2 (0x3be03000)
libidn.so.11 => /usr/lib/libidn.so.11 (0x3be18000)
libssl.so.0.9.8 => /usr/lib/libssl.so.0.9.8 (0x3be49000)
libcrypto.so.0.9.8 => /usr/lib/libcrypto.so.0.9.8 (0x3be92000)
libpulsecommon-0.9.19.so => /usr/lib/libpulsecommon-0.9.19.so (0x3bff3000)
libcreds.so.2 => /usr/lib/libcreds.so.2 (0x3c051000)
libresolv.so.2 => /lib/libresolv.so.2 (0x3c05e000)
libpixman-1.so.0 => /usr/lib/libpixman-1.so.0 (0x3c07d000)
libpng12.so.0 => /usr/lib/libpng12.so.0 (0x3c0fd000)
libXrender.so.1 => /usr/lib/libXrender.so.1 (0x3c126000)
libexpat.so.1 => /usr/lib/libexpat.so.1 (0x3c137000)
libpcre.so.3 => /lib/libpcre.so.3 (0x3c163000)
libXau.so.6 => /usr/lib/libXau.so.6 (0x3c1aa000)
libsndfile.so.1 => /usr/lib/libsndfile.so.1 (0x3c1b6000)

Are the missing libs available for Harmathan, oder would it make sense to copy the missing deps by hand from the N900 to the N9?
Thank you in advance.