firefox: disable content sandboxing - not ready yet

Details: https://github.com/NixOS/nixpkgs/issues/24295#issuecomment-300588575

Changed files
+1 -1
pkgs
applications
networking
browsers
firefox
+1 -1
pkgs/applications/networking/browsers/firefox/common.nix
···
"--enable-system-sqlite"
#"--enable-system-cairo"
"--enable-startup-notification"
-
"--enable-content-sandbox" # available since 26.0, but not much info available
+
#"--enable-content-sandbox" # TODO: probably enable after 54
"--disable-tests"
"--disable-necko-wifi" # maybe we want to enable this at some point
"--disable-updater"