Run Firefox inside an XTerm, it doesn't crash mysteriously this way.
Also try opening developer tools and checking that Firefox doesn't
crash in the process.
You can now run a test in the nixos/tests directory directly using
nix-build, e.g.
$ nix-build '<nixos/tests/login.nix>' -A test
This gets rid of having to add the test to nixos/tests/default.nix.
(Of course, you still need to add it to nixos/release.nix if you want
Hydra to run the test.)
