Video Preview (for YouTube™) Is an extension for Google Chrome - https://chrome.google.com/webstore/detail/youtube-video-preview/hmggjocpafbpjdhoknaneokaklekakeh

Extension change log can be found at: https://github.com/darkyndy/YoutubeVideoPreview/wiki/Google-Chrome-extension---change-log

To read more about the extension you can check wiki page: https://github.com/darkyndy/YoutubeVideoPreview/wiki (available only for Google Chrome extension)

FireFox Extension - not supported any more - https://addons.mozilla.org/en-US/firefox/addon/youtube-video-preview/