I've used Aurora for some time, extensions often get borked or don't work, so I went back to Beta.
btw one tip for people who switch to beta/aurora - disable extensions compatibility check.
There's an Arch developer named heftig who maintains a custom repository with Aurora. I've been using it successfully the last few months. I think aurora is updated daily, but I haven't watched it closely.
Info: https://bbs.archlinux.org/viewtopic.php?id=117157 Repo 32-bit: http://pkgbuild.com/~heftig/repo/i686 Repo 64-bit: http://pkgbuild.com/~heftig/repo/x86_64
Another option is to download the latest build [1] and use the built-in updater.
[1] https://ftp.mozilla.org/pub/mozilla.org/firefox/tinderbox-bu...
I've switched to Aurora for the past month and it's noticeably faster than FF18b on my older hardware.
I wish there was a reliable Aurora channel on Arch. The pkgbuild system isn't well suited to keeping up to date with a moving filename target like Firefox.