Update packages (#87)

* Update packages that require a non-major version bump

* Update ESLint to latest version

* Fix ESLint violations after ESLint upgrade

* Set eslint config as root config

* Test in Firefox instead of Chrome

Chrome 87 just shipped the CookieStore implementation and it's not
possible to null out the `window.cookieStore` variable. So we're
switching to test in Firefox which hasn't implemented the feature yet so
that we can continue to test.

Not a great solution but maybe we can find a better one in the future,
this should do for now.

* Actually use FirefoxHeadless and not Firefox

.eslintrc.json

@@ -1,4 +1,5 @@
 {
+  "root": true,
   "env": {
     "browser": true,
     "es6": true,