While most advice recommends that you install the 32-bit version of flash along with an emulation layer, I’ve found the 64-bit version of the flash player works better than the 32-bit version and is generally quite stable.
Adobe Labs released their alpha flash player 10 for 64-bit linux in November 2008. In order to install it, simply go to the download site, download the .tar.gz file and extract it. The uncompressed file is just the libflashplayer.so. In the install instructions they say to place this file in ~/.mozilla/plugins and restart Firefox. However, when I tried this I found it didn’t work. In order to make the plugin load, I placed it under /usr/lib/mozilla/plugins/ and this did the trick.