My Microsoft Comfort Curve 2000 keyboard has the usual gaggle of multimedia & program-control keys. I tend to not use many of those keys, but Volume +/- and Mute come in handy.
The default Arch Linux keyboard configuration, whatever it is, reports the appropriate codes from the keyboard, so I didn’t have to fiddle with any of that.
I used the same basic procedure as I described there, with tweaks to the amixer commands:
amixer sset Master,0 3dB+ amixer sset Master,0 3dB- amixer sset Master,0 toggle
Linking the keys and commands goes through the XFCE4 Keyboard settings applet (or whatever those things are called).
For what it’s worth, the Back and Forward keys work perfectly in Firefox, right out of the box.