User Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:68.0) Gecko/20100101 Firefox/68.0

Steps to reproduce:

1 - Unzip the attached file somewhere on the disk
2 - Unzip and install the extension contained into bug-webrequest-file.zip. This extension registers 1000 empty handlers by calling browser.webRequest.onBeforeRequest.addListener(() => { }, { urls: ["<all_urls>"] }, ["blocking"]); 1000 times
3 - Open the index.html page (from the attached zip file). This page contains hundreds of data URIs.

Actual results:

The browser is unresponsive.

Expected results:

The browser should not be unresponsive. This issue does not exist in Chrome. Moreover, it seems that this API does not work with data URI requests, e.g. I wasn't able to cancel them.

I'm confirming this, here is a sample requestDetails being sent:
{ "documentUrl": "moz-nullprincipal:{99fd9ed7-8845-4aa3-ab08-dafae7cf7bd1}", "frameAncestors": [], "frameId": 15032385550, "method": "null", "originUrl": "moz-nullprincipal:{99fd9ed7-8845-4aa3-ab08-dafae7cf7bd1}", "parentFrameId": 15032385537, "requestId": "fakeRequest-436", "tabId": 6, "timeStamp": 1559858482376, "type": "image", "url": "data:image/jpeg;base64,/9j/4AAQSkZJRgABAQAAAQABAAD....." }

Hi, I managed to reproduce this issue In our latest Nightly version 69.0a1 (2019-06-11) as well Release and Beta using the files attached to this issue. Thanks for reporting it and thanks for the test case provided.

The priority flag is not set for this bug.
:ddurst, could you have a look please?

For more information, please visit auto_nag documentation.

Eventually the socket process may help here. We can also be smarter about how we handle these requests.

The duplicate here is newer, but has more information and a concrete plan for addressing this.