background.js

var blockedRequestIds = new Set();

browser.webRequest.onBeforeRequest.addListener(
    function(details) {
        if ( details.url.includes('favicon') === false ) { return; }
        blockedRequestIds.add(details.requestId);
        console.log('blocking tabId=' + details.tabId + ' requestId=' + details.requestId + ' url=' + details.url);
        return { cancel: true };
    },
    { urls: [ '<all_urls>' ] },
    [ 'blocking' ]
);

browser.webRequest.onResponseStarted.addListener(
    function(details) {
        if ( details.url.includes('favicon') === false ) { return; }
        if ( blockedRequestIds.has(details.requestId) === false ) {
            console.error('onResponseStarted: favicon request not reported to onBeforeRequest: ' + details.requestId);
            return;
        }
        console.error('onResponseStarted: favicon request: ' + details.requestId);
        blockedRequestIds.delete(details.requestId);
    },
    { urls: [ '<all_urls>' ] }
);

browser.webRequest.onErrorOccurred.addListener(
    function(details) {
        if ( details.url.includes('favicon') === false ) { return; }
        if ( blockedRequestIds.has(details.requestId) === false ) {
            console.error('onErrorOccurred: favicon request not reported to onBeforeRequest: ' + details.requestId);
            return;
        }
        console.log('onErrorOccurred: favicon request: ' + details.requestId);
        blockedRequestIds.delete(details.requestId);
    },
    { urls: [ '<all_urls>' ] }
);

manifest.json

{
    "manifest_version": 2,
    "name": "umatrix-issue-925",
    "version": "1.0",
    "background": {
        "scripts": ["background.js"]
    },
    "permissions": [
        "webRequest",
        "webRequestBlocking",
        "<all_urls>"
    ]
}