Update of year 2014.
Now there is a way to get Accept-Languages in Firefox and Chrome using navigator.languages (works in Chrome >= 32 and Firefox >= 32)
Also, navigator.language in Firefox these years reflects most preferred language of content, not language of UI. But since this notion is yet to be supported by other browsers, it is not very useful.
So, to get most preferred content language when possible, and use UI language as fallback:
navigator.languages
? navigator.languages[0]
: (navigator.language || navigator.userLanguage)