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Waiting until the addon database is ready and the addonReady promise resolves can take far, far too long, and cause regressions in our Talos tests. What we really care about waiting for is resource-mapping.js having finished its job to map resource://newtab and chrome://newtab URIs to the right places. So we just have resource-mapping.js directly inform AboutNewTabRedirector when that has occurred. Differential Revision: https://phabricator.services.mozilla.com/D240939