wrapFirefox: move out of all-packages.nix, change defaults

- I don't think that amount of code belonged into all-packages.nix.
- Now the default name of the wrapped package is identical
  with the command that runs the browser.
- Other defaults were changed according to how the wrapper is
  (almost always) used.
- `meta` is improved: mostly inherited with priority above
  the unwrapped package.