Volume Control v2.0.2
If you're still on Windows 7, you'd probably have noticed a bug in Firefox where the system's Mixer would not remember the volume levels for new tabs. This is especially noticeable if you're wearing earphones. Volume Control is a fix for that. Bug or no bug, if you want a global volume slider to control the volume of HTML5 media, this is the extension for you.
v2.0.2
Added new feature -- instantaneous volume adjustment (no more page refreshes when adjusting volume)