Looks like this is because of an error in https://phabricator.services.mozilla.com/D138503 . The correct usage of the [early beta or earlier](https://searchfox.org/mozilla-central/rev/94d7c959115c03ea1e9406d6105b36cabe63775d/modules/libpref/init/StaticPrefList.yaml#159) constant in StaticPrefList.yaml is `@IS_EARLY_BETA_OR_EARLIER @`.
Bug 1755457 Comment 2 Edit History
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Looks like this is because of an error in https://phabricator.services.mozilla.com/D138503 . The correct usage of the [early beta or earlier](https://searchfox.org/mozilla-central/rev/94d7c959115c03ea1e9406d6105b36cabe63775d/modules/libpref/init/StaticPrefList.yaml#159) constant in StaticPrefList.yaml is `@IS_EARLY_BETA_OR_EARLIER@`.