remove firefox integration tests from dist build owing to flakiness

[trevj]

They're too flaky and I'm trying hard to make the release process to be as smooth as possible.

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This change is

[trevj]

Also, uproxy-lib's smoke and manual tests exercise all this code anyway.

[jpevarnek]

Aww, sad

👍 though

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Review status: all files reviewed at latest revision, all discussions resolved.

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