User Agent: Mozilla/5.0 (X11; Ubuntu; Linux i686; rv:46.0) Gecko/20100101 Firefox/46.0 Build ID: 20160425115337 Steps to reproduce: Return {cancel: true} from a `webRequest.onBeforeRequest` listener which logs each call to it in the console. (In Firefox 46 on Ubuntu 14.04.) Actual results: Two messages are logged from the `onBeforeRequest` listener if it returns {cancel: true} to block the load, and one is logged otherwise. That indicates that the listener is called a second time when it cancels a request. If the request is canceled, an error follows the two messages, but the load is terminated. The console output: onBeforeRequest: blocking bug 1271553 background.js:9:3 >>>twice; this is my extension reporting. Then,<<< [Exception... "Component returned failure code: 0x805e0006 [nsIWebNavigation.loadURIWithOptions]" nsresult: "0x805e0006 (<unknown>)" location: "JS frame :: chrome://browser/content/browser.js :: _loadURIWithFlags :: line 839" data: no] browser.js:860:0 Expected results: The `webRequest.onBeforeRequest` listener should have been called just once, and the load terminated without error. The attached WebExtension demonstrates the problem. `onBeforeRequest` listens for Bugzilla bug report URLs, blocks odd-numbered bugs, and logs whether it blocked or passed each page. In Chrome, one message is logged from `onBeforeRequest` for each URL, and URLs are blocked without error. In Firefox, two messages are logged if a URL is blocked but one message if it isn't, and an error follows the two messages when a URL is blocked.

I think this is the same issue as bug 1272199. Please reopen if you can still reproduce on Firefox 48.