If you are working on a x86_64 system don’t even try to go with 64bit of FireFox, just install 32bit version and enjoy all FireFox’s plugins: flash, java, … don’t waste your time.

Adobe didn’t released yet a 64bit flash plugin for linux, although there exists different workarounds to make 32bit plugins to work on the x86_64 platform they are still buggy.

If you’ll try to use nspluginwrapper you’ll end up with FireFox eating 1GB of ram after you’ll open a few flash sites because it’s leaking memory.

Let’s go to install firefox with flash-plugin, firstly remove firefox x86_64 version and then install i386 version:

[root@silver ~]# yum remove firefox.x86_64
[...]
[root@silver ~]# yum install firefox.i386
[...]

Then we’ll download rpm for linux from Adobe :
http://www.adobe.com/shockwave/download/alternates/

[root@silver ~]# rpm -ivh flash-plugin-9.0.124.0-release.i386.rpm
[...]

Now, you are ready to use your favorite browser on linux x86_64 with flash player.

Update:

An alpha refresh of 64-bit Adobe Flash Player 10 for Linux operating systems was released on 12/16/2008.

Download it from here:
http://labs.adobe.com/downloads/flashplayer10.html