steamos is based on Arch Linux using Bazzite, an Atomic version of Fedora. Fedora "Atomic" Desktops have an immutable read-only core system.

Installing Firestorm official OpenSimulator Linux version works, however it blocks on dialog/filechooser, probably related to GTK portal services. What happens when you use "Import Image" or mesh, etc, the viewer blocks and the chooser never opens. Pressing "Control+C" allows the chooser to load but then shuts down Firestorm.

One work-around is installing Fedora 42 using Distrobox solves the blocking problem however the chooser dialog remains open after selecting a file despite pressing "ok" or "cancel".
Distrobox

distrobox create --name firestorm42 --image registry.fedoraproject.org/fedora:42
distrobox enter firestorm
sudo dnf install \
glibc libstdc++ mesa-libGL mesa-libGLU \
alsa-lib pulseaudio-libs libXcursor libXi libXrandr \
libXinerama libXrender libXScrnSaver \
gtk3 dbus-glib atk cups-libs nss nspr \
fontconfig freetype

Run in Distrobox

cd ~/Downloads/Phoenix-FirestormOS-Releasex64_AVX2-7-2-4-80712
./firestorm

Flatpak

The best solution is to create a Flatpak repo to install on the streamos/Bazzite host.

Holoneon OpenSim Firestorm (Flatpak)
https://github.com/holoneon/org.holoneon.FirestormOpenSim

you can build and run yourself now, but will be added to Flathub for easy one-click install soon
if you want your grid listed in the drop-down list visit:
https://github.com/holoneon/opensim-grid-directory