Open pageAction instead of popup on Firefox

Firefox now allows extensions to [open the pageAction programmatically](https://developer.mozilla.org/en-US/Add-ons/WebExtensions/API/pageAction/openPopup), which is the view that people see when they click the fox.

We've kindof wanted this forever! Makes it very obvious when the confirmation is from MetaMask, vs an impostor popup from another page, since it has UI that overlaps with the browser itself.

Start with Firefox's pageAction.openPopup documentation and trace the extension's existing pageAction and popup entry points. The work is done when Firefox confirmations open the browser-integrated pageAction view instead of a page popup, while the existing confirmation behavior remains intact.

Written by the indexing model from the issue text.