Alt+Left and Alt+Right are the defaults shortcuts for going back and forward in the history in Firefox. Besides that, I didn't find a way to modify or remove whatever Xfce is doing with this shortcuts.
I believe this is a Xfce error because it doesn't happen in gnome (installed just to test this).
I'm using Arch.
PD: I googled around, tried some things that I found on the web, but I had no success, so this is very frustrating, and I guess not only for me.
With the default configuration, xfwm4 does not grab neither Alt+Left nor Alt+Right.
It works just fine here, so something else is grabbing those keys on your setup.
As Olivier indicated please check the shortcuts in your keyboard configuration.
So for Xfwm4 this report is invalid.