Motivation for this change

• Firefox Screenshots
• Address form autofill (en-US only)
• Updated Preferences
• Media opened in a background tab will not play until the tab is selected
• Improved Send Tabs feature of Sync
• Other improvements

https://www.mozilla.org/en-US/firefox/56.0/releasenotes/

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option build-use-sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • Linux
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Fits CONTRIBUTING.md.

I could not build the source package firefox. Here is a WIP branch:
https://github.com/taku0/nixpkgs/tree/firefox-bin-56.0

The probrem seems to be related to Rust and Clang.

/tmp/nix-build-firefox-unwrapped-56.0.drv-0/firefox-56.0/obj-x86_64-pc-linux-gnu/dist/system_wrappers/cert.h:3:23: fatal error: cert.h: No such file or directory
 #include_next <cert.h>

/tmp/nix-build-firefox-unwrapped-56.0.drv-0/firefox-56.0/obj-x86_64-pc-linux-gnu/dist/include/mozilla/Compiler.h:49:12: fatal error: 'cstddef' file not found, err: true
/tmp/nix-build-firefox-unwrapped-56.0.drv-0/firefox-56.0/obj-x86_64-pc-linux-gnu/dist/include/mozilla/mozalloc.h:16:12: fatal error: 'new' file not found, err: true
/tmp/nix-build-firefox-unwrapped-56.0.drv-0/firefox-56.0/obj-x86_64-pc-linux-gnu/dist/include/mozilla/mozalloc.h:16:12: fatal error: 'new' file not found, err: true

I cannot dig it deeper until the next Sunday. Please merge this firefox-bin branch. firefox-bin works fine.