FWIW I just noticed (looking at [this passing Playwright test](https://github.com/microsoft/playwright/blob/b9c0b92894916aae48aa200f0efd47dfac353514/tests/page/page-event-popup.spec.ts#L96-L103)) that Firefox sends the `script.realmCreated` event if the BrowsingContext was opened using `window.open(url, null, 'noopener')` but not when it was opened with `window.open(url)`.
Bug 2002721 Comment 5 Edit History
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FWIW I just noticed (looking at [this passing Playwright test](https://github.com/microsoft/playwright/blob/b9c0b92894916aae48aa200f0efd47dfac353514/tests/page/page-event-popup.spec.ts#L96-L103)) that Firefox sends the `script.realmCreated` event if the BrowsingContext was opened using `window.open(url, null, 'noopener')` but not when it was opened with `window.open(url)`. Likewise, clicking `<a target=_blank rel=noopener href="...">` will send that event but clicking `<a target=_blank rel=opener href="...">` won't.