Fix duplicate panels created in Firefox

[bgins]

Description

This PR implements the following fixes:

• [x] Prevent the creation of multiple panels
• [x] Add missing page load content script to Firefox
• [x] Generalize localhost permissions to all ports

Link to issue

Closes #16

Type of change

• [x] Bug fix (non-breaking change that fixes an issue)
• [x] Refactor (non-breaking change that updates existing functionality)

Test plan (required)

Temporarily load the extension into Firefox. Test it with a site that uses Webnative. When inspecting messages, only a single panel should be created.

The connection should remain across page reloads.

The extension should work on any localhost port.