Lyrics.com

The Lyrics.com extension allows you to search for lyrics directly from your browser anywhere on the web! To use this extension simply select any text you'd like to search lyrics for and click on the extension icon. A pop-up window will open with all possible song lyrics suggestions for the selected term -- alternatively, when you're on a YouTube song page, the extensions will automatically match the lyrics to the song you're listening to! Note: The extension will not work in tabs that were open prior to installation. After installing this extension, either reload your open tabs or restart Chrome.

Screenshots from Chrome Web Store

Screenshots from Mozilla Add-ons