So, I manage to get flash working on FreeBSD. Without using the Linux compatability layer!
Many thanks to drizzt for his precious suggestions.
I used graphics/gnash and www/firefox. Be sure not to check “GSTREAMER” on the gnash port configuration ncurses window: it didn’t work for me.
To watch YouTube videos, you must install multimedia/mplayer and www/mplayer-plugin, the Greasemonkey plugin for Firefox and install this script on Greasemonkey. Be sure to select the “mini” GUI for mplayer because it is needed for the plugin (update: be sure to install mplayer WITH_GUI (thx to Oliver for the hint)).
You should be done, or at least, that’s how I did it.