With my Web Storage example, things work great in Chrome and Safari but nothing happens in Firefox 5 on the Mac - even though I know the browser supports this feature.
The issue lies in the security model behind Firefox. It is quite restrictive in what it will let you do with file:/// urls. The intention is to avoid any chance of a remote page being able to access a local file.
That is fine, but there is no obvious alert or notification that Firefox is doing this - instead your code just doesn't work. If I hadn't run it on Chrome first, my reaction would have been to question my own code.
So bear that in mind if you work the same way that I do - perhaps use Chrome as your first choice for development - and if something simple doesn't work, try it in a different browser before questioning your own code.