fix: Origin in API calls in Firefox-65.0a1

[lidel]

Closes #622

Context: #622 (comment)

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FUN FACT: underlying URL.origin value depends on actual runtime

Node 8:

new URL('moz-extension://0f334731-19e3-42f8-85e2-03dbf50026df/').origin === 'null'

Firefox Firefox-65.0a1:

new URL('moz-extension://0f334731-19e3-42f8-85e2-03dbf50026df/').origin === 'moz-extension://0f334731-19e3-42f8-85e2-03dbf50026df'

Tests and code is value-agnostic, but it is worth being aware that url spec and implementations often diverge around edge cases.