There's a separate define for early beta: https://wiki.mozilla.org/Platform/Channel-specific_build_defines#EARLY_BETA_OR_EARLIER In your case, you'd want to re-land with @IS_EARLY_BETA_OR_EARLIER@ instead of @IS_NIGHTLY_BUILD@. That'd get you the first half of the Beta cycle before being automatically turned off.
Bug 1737405 Comment 34 Edit History
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There's a separate define for early beta: https://wiki.mozilla.org/Platform/Channel-specific_build_defines#EARLY_BETA_OR_EARLIER In your case, you'd want to re-land with @IS_EARLY_BETA_OR_EARLIER@ instead of @IS_NIGHTLY_BUILD@. That'd get you the first half of the Beta cycle before being automatically turned off. I don't think it'd be particularly risky to go ahead and land that change, but we should probably have a better sense of what to be on the lookout for before letting it ride the trains past early beta.