Add Firefox Support

[bramus]

With Firefox ramping up development again, it’s time to look into enabling the extension in Firefox.

This was blocked on https://bugzilla.mozilla.org/show_bug.cgi?id=1852246, but since filing the bug a solution/workaround was posted to it.

[bramus]

While I did manage to work around the looping problem as detailed in https://bugzil.la/1852246#c5, there are still other bugs in Firefox’s DevTools Extensions support that are blocking this:

1. useContentScriptContext for devtools.inspectedWindow.eval is not supported – https://bugzil.la/1852242
2. window argument to devtools.panels.ExtensionSidebarPane.onShown listeners is undefined – https://bugzil.la/1946904