I assume, you mean for the Live Plop Linux.
Download Firefox for Linux (32bit or 64bit depending on what you need) from
https://www.mozilla.org/en-US/firefox/all/Extract the archive.
You have to uncompress the file ploplinux/opt.sqfs
unsquashfs opt.sqfsChange to the directory 'squashfs-root' and remove the 'firefox' directory.
Copy the extracted 'firefox' directory from the downloaded Firefox archive to the 'squashfs-root' directory.
Remove the old 'opt.qfs'.
Compress the 'squashfs-root' directory.
squashfs squashfs-root opt.sqfsBest regards
Elmar