w3m is not capable of handling Javascript at all. And you know what, for 90% of the websites I visit, it doesn't matter. They function fine and look fine without Javascript. And if w3m could manage to make most websites look fine without Javascript, so could Firefox -- if its developers cared.
As for non-technical users -- they're probably not going to be opening Firefox's Preferences dialog in the first place. And if they do, they probably aren't going to start randomly checking and unchecking stuff to see what it does. That's something an adventurous geek might try, but certainly not your typical non-technical user.
If Firefox developers wanted to additionally protect the average user from this dangerous button, they could have simply stuck it in the Advanced tab of the Preferences dialog, or added a scary warning about being doubly sure that the user knows what he's doing (like they do with about:config).
That said, I'm happy to use NoScript for this functionality anyway, as it's far more flexible than a blanket "turn off Javascript everywhere with no exceptions" button.