chore: Replace Juggler with Firefox Nightly in CI

[mjzffr]

Fixes #5150

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[mjzffr]

Will be able to remove this bit once #5151 is fixed.

[mathiasbynens]

ack

[mathiasbynens]

Excited to see this happening! Thanks for your work on this :)

[mathiasbynens]

Let me know if there's a specific deprecation message/link you'd like to add to puppeteer-firefox.

[hanselfmu]

LGTM as well :)

[mjzffr]

Let me know if there's a specific deprecation message/link you'd like to add to puppeteer-firefox.

@mathiasbynens Will do. I hope to get to that soon.

[mjzffr]

I'll follow up in issue #5397

[whimboo]

@mjzffr, what is left to do here?

[mjzffr]

@mjzffr, what is left to do here?

@whimboo Nothing on my end, as I understand it. I assume @mathiasbynens et al will merge when they are ready.