`MOZ_ASSUME_USER_NS=1` envvar prevents librewolf from loading any tabs

Setting the envvar `MOZ_ASSUME_USER_NS=1` prevents Librewolf from loading any
tabs and causes it to spam the console output with
```
[Parent 2, IPC Launch] WARNING: fork() failed: Operation not permitted: file /root/.local/share/bsys6/work/librewolf-118.0.1-1/ipc/chromium/src/base/process_util_linux.cc:280
[Parent 2, IPC I/O Parent] WARNING: Failed to launch tab subprocess: file /root/.local/share/bsys6/work/librewolf-118.0.1-1/ipc/glue/GeckoChildProcessHost.cpp:787
```
Note that this issue does not occur on a regular (non-flatpak) Librewolf, so it
is something about how this package that is causing the problem; maybe it's
something with the permissions?

As a workaround, unsetting the variable `MOZ_ASSUME_USER_NS` through flatpak
overrides seems to work for me.