Well, if you already have flash and stuff set up for another mozilla-y browser, which you're not going to get rid of, you can just symlink the plugins directory of that to the plugins directory of firebird.. works for me.
If not... What did extracting the tar.gz give you? you might need to make/make install, i can't remember. Anyway, by the end you want a readable this in your plugins directory: a readable flashplayer.xpt and an executable/readable libflashplayer.so. That should be it, just restart firebird and check, put "about
lugins" into the location/address/whatever bar to check installed plugins.
There's no special procedure for firebird. Mozilla plugins should work for it.