Firefox 4.0 final will be released tomorrow officially, but it seems like the setup has already been made available on Mozilla ftp servers, and some download sites.
I personally saw the download available on www.kaldata.net
The download on the Mozilla Firefox web page will be available after the official announcement tomorrow. But, for the impatient ones, who want to download and install the final version, they can do so by visiting Mozilla ftp servers, or download sites like kaldata, and Softpedia.
More details are available on this Ghacks article page :
I personally, will wait for the download to be available on the Mozilla official Firefox page. Well, its finally here