Many Linux OS doesn't contain the Firefox up to date repository, even some only support 3.6.X version. On the other hand, firefox released new version every few weeks and the performance improved a lot especially the latest 15.0 because the memory leaks problem is fixed since this version. Here is some tips for installation of latest firefox:
1. download either source code or compiled version from
ftp://ftp.mozilla.org/pub/mozilla.org/firefox/
2. extract .tar file into local folder such as: /soft/firefox
and make the following changes for your system so that the latest version can take effect: