I wrote a very simple web extension for Firefox (92.0 on Windows 10).

manifest.json:

{
    "manifest_version": 2,
    "name": "Sandbox",
    "version": "1.0.0",
    "description": "my sandbox for a Firefox WebExtension",

    "icons": {
        "16": "icons/dlwatch16.png",
        "32": "icons/dlwatch32.png"
    },
    "background": {
        "scripts":  [ "script.js" ]
    },
    "permissions": [ "<all_urls>", "webNavigation"]
}

script.js:

browser.webNavigation.onBeforeNavigate.addListener((details) => {
    console.log(`browser.webNavigation.onBeforeNavigate.listener: details: ${JSON.stringify(details)}`);
});

I start Firefox for a test with "web-ext run -v" that starts a Firefox window. I open the console of Firefox with the menu of the window. When entering https://www.mozilla.org as the URL if the tab, I get in the console:

browser.webNavigation.onBeforeNavigate.listener: details: {"url":"https://www.mozilla.org/","timeStamp":1632494436204,"frameId":0,"parentFrameId":-1,"tabId":1,"windowId":3}

But with the URL "about:addons", The result in the console is:

browser.webNavigation.onBeforeNavigate.listener: details: {"url":"about:addons","timeStamp":1632494444353,"frameId":0,"parentFrameId":-1,"tabId":1,"windowId":3} script.js:2:10 browser.webNavigation.onBeforeNavigate.listener: details: {"url":"about:addons","timeStamp":1632494444416,"frameId":0,"parentFrameId":-1,"tabId":1,"windowId":3} script.js:2:10

Does someone know why the listener is called twice for about:addons ?