Show an explanation for mozilla domains instead of an error.

[532910]

#62 (comment)

Browserpass should show an explanation about script injection blocking on mozilla domains instead of "Error: Error: Unable to inject script in the top frame".

[max-baz]

Not sure if this warrants a code change or simply a mention in README (would be nice to avoid browser-specific code as much as possible), but yes, definitely should be more discoverable.