If you already set up Sync, and Firefox is connected, your user name is displayed on the Options/Preferences page. Either:

• "3-bar" menu button (or Tools menu) > Options
• type or paste about:preferences in the address bar and press Enter

In the left column, click Sync. Then your username should be displayed along with your email address on the right side.

I should correct that by saying your email address is your username for login purposes, and what appears above it is your "display name".