Go to Mozilla.org and download the latest version. I use MozillaFirebird so I'm not sure if the instructions are the same for Mozilla 1.4 or not. Installation instructions are available at the website.
Untar the file. If it's the same as Firebird, that's all you have to do. It will create a new directory. Go into that directory and click on the shell script to start the new version. The icon will look like a sea shell.
If everything works, delete the older version.