Bug 1123517 adds a BrowserUITestUtils.jsm testing module for use in tests, and implements waitForEvent. Numerous test files and head.js files have their own duplicate implementation of this function (waitForEvent/promiseWaitForEvent). Those can now be removed, in favour of importing BrowserUITestUtils.jsm and using that. It can be imported as resource://testing-common/BrowserUITestUtils.jsm
As discussed in Bug 1093566, BrowserUITestUtils is going to be migrated into the testing directory and directly introduced into mochitest-browser scope (combined with BrowserTestUtils).
Summary: Use BrowserUITestUtils.waitForEvent() in /browser/ tests → Use BrowserTestUtils.waitForEvent() in /browser/ tests
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