Test native Firefox implementation on CI too

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• https://github.com/puppeteer/puppeteer/issues/5743#issuecomment-621664876

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Impractical 😩

• https://github.com/mozilla/web-ext/issues/1927
• https://github.com/mozilla/web-ext/issues/2439
• https://github.com/puppeteer/puppeteer/issues/5532#issuecomment-706239839
• https://github.com/pixiebrix/pixiebrix-extension/issues/534

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Package deprecated. Never gonna happen.