[Firefox] Background page is null when shared with private window

[underrobyn]

An issue has arose as a result of how Firefox handles separating private and non-private windows.

When running Trace in private mode, the methods:

chrome.runtime.getBackgroundPage(function(bg) { });
// and
chrome.extension.getBackgroundPage();

will return null for private windows.

To prevent this, we need to change the report and blocked pages over to a message passing communication system which is used by the new settings page. Since the options page would require a lot of modifications, for now we will leave it and recommend that users change Trace settings in a non-private window.