I am new to Porteus, but I believe that you can download the latest version of Firefox from the official website, and use Porteus Modules (under the system tab in the menu) to covert this to a .zxm module. Then you would have to move the module to porteus/modules/ and activate it.
I hope this helps.
Hello. I am having trouble installing v4.0, as it fails with error code '3' when I execute the 'Porteus-installer-for-Linux.com' shell script installation file. I tried this on two linux distros and obtained the same error. Here is some lines of code from the terminal. Installing Porteus to /dev/sdb...