A Greasemonkey script for Firefox called "Google Music Lyrics" helps you adds a lyrics panel to Google Music beta.
This script adds a panel on the right in Google Music Beta for lyrics. Lyrics are fetched from SongLyrics.com and automatically populated whenever you change songs. It wouldn't take much to alter the script to work with other Lyrics websites. Right now, it just does a search based on title and author and selects the first result.
The lyrics are cached so that they're displayed instantly the next time you play the same song. You can also edit the lyrics and the changes are saved using HTML5 local storage.
To hide the panel, click the title.
Chris Hendry, who developed the script, plans to add other lyrics providers, allow users to import and export lyrics and to make the script work in Google Chrome.