First update your environment
Open a terminal and type the following:sudo apt update; sudo apt upgrade; sudo apt autoremove
Then Download Firestorm from their web site.
https://www.firestormviewer.org/choose-your-platform/
Pick the Penguin, and the 64 bit download.
Extract the file by either tar -xvf Phoenix-Firestorm-Releasex64-6-6-8-68380.tar.xz or with the file viewer using Archive Manager.
Now install. I use /opt/fire to install to.
Open a terminal again.
cd Phoe*68380
sudo ./install.sh
If you would prefer native linux SL Voice, you will need to install a couple of i386 library files.
sudo apt-get install gstreamer1.0-pulseaudio:i386 libidn12:i386 libuuid1:i386 libstdc++6:i386
You will also need to create a symbolic link from libidn12 to libidn11 in your /lib/i386-linux-gnu directory.
cd /lib/i386-linux-gnu
sudo ln -s libidn.so.12 libidn.so.11
You will not need to add these files or recreate the symbolic link for subsequent installs of firestorm.
Firestorm should now run with linux native SLVoice support.