Just like the title states. How come this setting is disabled (2 in about:config rather than 0) under Linux by default? Is there a reason to not turn it on?
It's turned off because that's just not something that browsers under Linux do/use. Rather than a write-once-run-anywhere deal, Firefox is tuned for each platform it's built for, in menus, locations, and actions.
Having said that, you can go ahead and set it to something if that's what you like.