I place all plugins in /usr/lib/mozilla/plugins. Works fine for me when messing with 64bit firefox. Just went back to 32bit firefox on 64bit OS for better Java support.
Distribution: RHEL 5.1 with Pieces of this and that.
Kernel 188.8.131.52, KDE 3.5.8 and KDE 4.0 beta, Plus Development src.rpm, ATI fglrx64_7_1_0-8.433-1 rpm with 3D and DRI working.
Acer 5100-5840 with webcam, ati, sdcard reader, sound, atheros based wireless, all working. Only thing not working is the memory stick reader.