Edited for wording/formatting, and then edited a second time because I forgot an important step for screen casting.

Hiya! In case you're still having some trouble, here is a step-by-step written guide. If you still want a video tutorial, I can definitely make you one! If you have any trouble after following these steps, let me know for sure. I can try to help as best I can!

First of all, relevant to what you want to use this extension for, this won't work. When you cast YouTube, you will still get the ads because it launches from the app on the Chromecast. Unfortunately, casting the screen doesn't seem to cast the audio. So casting from a browser with an ad-block, you won't be able to watch shows unless you have the sound coming from your laptop. If I find out that it is supposed to and get it set up, I will update you on that,

The best way to stream to a Chromecast and cast the audio at the same time is to use the Google Home app on a mobile device, and cast your screen that way. Big downside of course, is that you have to have it connected to your account. If you already have a YouTube account though I assume that's fine. It won't be able to track what you are sharing on your screen. Just make sure you have the relevant ad-block extension installed on the mobile browser of your choice!

Installing the extension:

1. Go to: https://hensm.github.io/fx_cast/
2. Click on the "Windows Bridge" button and download it. Install that. Windows may pop up telling you it's suspicious. Click "see more," and click "install."
3. During the install, it will ask if you want to install dependencies. DO install the dependencies.
4. Go back to the same page that you just downloaded that from.
5. Click on "Firefox Extension" button, and follow the steps to add that extension.

For Media Websites

1. Complete the above steps.
2. Select the puzzle shape in your extensions toolbar, click "manage extensions." This can also be opened by typing about:addons into the URL bar.
3. In this menu, at the bottom, you can add sites to the whitelist to allow casting them. These can only be websites with a corresponding app associated with them. There is a dropdown to select from some of these options. Click one of them, and click the + sign to add it. Note: "All 4" is a website for the Channel 4 website. It's not a way to add any particular 4 services.
4. When on a whitelisted website with a compatible Chromecast app, there may be a "cast" option in the video player itself. You may use that, or right-click (on a part of the page outside of the video player) and select "cast." A small window should pop up with the names of your Chromecast devices. A small window will pop up.
5. There is a dropdown menu in between "cast" and "to:" Make sure the dropdown says the name of the website (ex. YouTube). If you don't see it, return to step 3 to whitelist it.
6. Click "cast" next to the device you want to cast to.
7. It should cast to the device! If it doesn't, post a screenshot/photo of the problem and I can try to help you out.

For Screenshare

1. Complete the steps for Installing the Expansion.
2. You don't have to go to the extensions option for this one!
3. Right click anywhere on your screen, and select the "cast" option.
4. There is a dropdown menu in between "cast" and "to:" Make sure the dropdown says "screen."
5. Click "cast" next to the device you want to cast to.
6. Another window will pop up to confirm the connection.
7. Click on the "source" button.
8. Select the window you would like to share. You may also select "entire screen" if you don't want only a specific window.
9. Click on "allow."
10. A small floating taskbar will appear with a "stop sharing" button. You can click that to, well, stop sharing. You may also minimize it to get it out of the way by clicking the - sign on the right.
11. It should cast to the device! If it doesn't, post a screenshot/photo of the problem and I can try to help you out.
12. *I don't think it casts audio at this point for screenshare. That's the only downside.