Experiment with new Firefox methods of providing an API from extension

Firefox has a format for defining a "Browser experiment" which can be an experimental extension API.

One of our longest standing criticisms is our extreme browser permissions, and we've long said we could dramatically reduce that permission's scope if there was a "Allow this extension to provide a voluntary web service to participating web sites" type permission.

The latest Firefox 59 beta (and later) provides the ability to bundle a web experiment with an extension, so we could start trying out a different way of providing the web3 API to the browser, in a way that reduces our potential for compromise.

Relevant links:
https://blog.mozilla.org/addons/2018/01/26/extensions-firefox-59/
https://bugzilla.mozilla.org/show_bug.cgi?id=1364976
https://bugzilla.mozilla.org/show_bug.cgi?id=1323845

No repository files, tests, or entry points are named. Start by reading the linked Firefox 59 announcement and Bugzilla issues 1364976 and 1323845, then compare the proposed browser experiment with the extension's existing web3 API; completion criteria are not defined beyond experimenting with a reduced-permission approach.

Written by the indexing model from the issue text.