According to this Reddit post, the following filter prevents thumbnail images from covering the screen during the final 20 seconds of a YouTube video.
- Click the AdBlock toolbar icon and then the gear symbol.
- On the Options menu, select the Customize tab.
- Click Edit to open the custom filters box.
- Paste the filters below exactly as given.
- Click Save.
- Reload the page you're trying to view.
Copy this filter:
youtube.com##.ytp-ce-element
If that doesn't work, try these:
youtube.com##.ytp-ce-covering-overlay
youtube.com##.ytp-ce-element-shadow
youtube.com##.ytp-ce-covering-image
youtube.com##.ytp-ce-expanding-image
youtube.com##.ytp-ce-element.ytp-ce-video.ytp-ce-element-show
youtube.com##.ytp-ce-element.ytp-ce-channel.ytp-ce-channel-this
If it doesn't help, clear your browser cache and cookies, reset your browser settings, and make sure your browser is up-to-date.