Prior to Bug 1476333, we would access the browser chrome URL from the frontend in the following ways:
1) Hardcoded "chrome://browser/content/browser.xul"
2) Hardcoded "chrome://browser/content/" which redirects to "chrome://browser/content/browser.xul"
3) getBrowserURL() from utilityOverlay.js which returns (1)
4) Services.prefs.getCharPref("browser.chromeURL") which returns (2)
These have now been replaced by a single preprocessor directive which is available via AppConstants. If you need to reference browser.xul, please use `AppConstants.BROWSER_CHROME_URL` instead. This makes it easier to find and fix issues with loading the browser window as an HTML document since the URL only needs to change in one place.
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