FireFox support

[hymccord]

Could you upload a version to the FireFox store? I've built and tested locally and it works fine.

Edit: It runs, but does have trouble saving the tables for some reason.

Only two changes are required for FF MV3 support:

1. Background script declaration still uses the MV2 style

  "background": {
    "scripts": ["background.js"]
  },

2. FF MV3 addons must have their id specified in manifest.json. MDN reference

"browser_specific_settings": {
  "gecko": {
    "id": "addon@example.com",
    "strict_min_version": "42.0"
  }

Happy to submit PR if you want!