Well if you insist on using FF then for now you would have to make due with Live Reload. Though of course as the name suggests it reloads the page, but there is no server required.
You could also use something like Brackets which has this functionality, but it only works in Chrome atm.
A final option is Emmet LiveStyle which works much like Brackets but doesn’t come with a code editor that you have to use. It works in Chrome and Safari.