That is the only one I could find. En will have to do for now. I also forgot to mention that you have to do the Help->Check for Updates. To actually get the 3.1b4 version as an update.

This might tide you over.

ftp://ftp.mozilla.org/pub/mozilla.org/firefox/tinderbox-builds/mozilla-1.9.1-linux/1236876773/firefox-3.1b4pre.en-US.linux-x86_64.tar.bz2