[Firefox] Enable linting of *.jsm files and update the code style in extensions/firefox/content

By default JSHint will only lint `*.js` files, however it is possible specify that other file types should be covered as well. To ensure a consistent code style, and to help catch errors, I think that it's a good idea to also include `*.jsm` files when running lint.
Besides updating the `node make lint` command, this PR also makes the necessary adjustments to make linting pass.

Besides, obviously, testing this using the built addon, I've also made extensive use of the addon debugger to make sure that changing to strict equalities doesn't change the behaviour.

*Note:* In a couple of the files I've relaxed the maximum line length requirement, because enforcing 80 characters would have meant adding a, in my opinion, disproportionally large number of new lines. I tried to keep the total diff as small as possible, but I would be happy to change this is if that is preferred.