Obtain cookies.txt for yt-dlp without external software or browser extensions. Tested with a Firefox private browsing session.

yt-dlp cookies.txt for YouTube.js

// This work is marked CC0 1.0 Universal.
// To view a copy of this mark, visit https://creativecommons.org/publicdomain/zero/1.0/ 

// 1. Watch a video
// 2. Open the network inspector, and pick a request to the www.youtube.com domain
// 3. Copy the value of the Cookie: request header and store it into a string
//    variable named "cookies" on the JavaScript console
//      cookies = "name=value; ..."
// 4. Run the code below to obtain the contents of cookies.txt
// 5. Pass it to yt-dlp with yt-dlp --cookies /path/to/cookies.txt

const lines = cookies
    .split("; ")
    .map(c => {
        const [, name, value] = c.match(/^([^=]+)=(.*)$/);
        return [
            ".youtube.com",
            "TRUE",
            "/",
            "TRUE",
            Number.MAX_SAFE_INTEGER,
            name,
            value
        ].join("\t");
    })
    .join("\n");
console.log("# Netscape HTTP Cookie File\n" + lines);