Flatpak: cannot run Firefox Nightly

[lemmarathon]

Issue description

Running Firefox Nightly produces the error message:

bwrap: Unexpected capabilities but not setuid, old file caps config?

The bwrap binary is not setuid.

$ ls -l /nix/store/a54avihgaaq1sqzr4rw3i3x93vp5iyak-bubblewrap-0.2.1/bin/bwrap
-r-xr-xr-x 2 root root 60896 Dec 31  1969 /nix/store/a54avihgaaq1sqzr4rw3i3x93vp5iyak-bubblewrap-0.2.1/bin/bwrap

Should this be changed in the bubblewrap package?

Steps to reproduce

1. Install flatpak-0.11.7 and bubblewrap-0.2.1
2. Install the Firefox Nightly flatpak from https://firefox-flatpak.mojefedora.cz/
3. $ flatpak run org.mozilla.FirefoxNightly

Technical details

• system: "x86_64-linux"
• host os: Linux 4.16.9, NixOS, 18.09pre140507.70c60a2fdca (Jellyfish)
• multi-user?: yes
• sandbox: yes
• version: nix-env (Nix) 2.0.2
• channels(root): "nixos"
• channels(lemmarathon): ""
• nixpkgs: /nix/var/nix/profiles/per-user/root/channels/nixos/nixpkgs

[lemmarathon]

Upon further testing, this seems to be a Sway bug, as Nightly launches fine in Weston and GNOME. When running in Sway, a Web Content process is created, but no GUI appears. Perhaps related to swaywm/sway#571? Issue seems to be resolved upstream in the wlroots branch. Avoid running in Sway as a temporary workaround.