firefox 4 has only been out 2 days, i would imagine only the very latest extensions to be compatiable with it. Even more so considering it can take upto 1 week for updates to an extension to be approved by mozilla.
With the new meter it'd be nice if it could be put up in the bookmarks toolbar, and only show a small icon. A small icon is key, don't want a huge bar taking up space (could perhaps be enabled if desired). Don't really want to have to re enable the bottom status bar thing, only for the usage monitor either.