Mozilla has finished working on Firefox 13 and marked it as Final, even though they haven't announced it officially and give some time to mirror site to fetch the latest version from their FTP server and also to Linux vendors to build a customized package for their own distribution.
If you can't wait that long, you can just go to their FTP Server and download the suited version according to your platform. Please note that you will have to download the whole package instead of downloading a delta. If you have used a binary version of Firefox previously, i would suggest to wait and use the Automatic Updates mechanism built in into Firefox. This way, you won't have to download the whole build, but only the delta, which is smaller in terms of size.
If you are curious about what's new on this release, check the Release Notes