Firefox
Click the menu button > Help > About Firefox



or for older versions:
Click Help on the menu bar > About Firefox


Chrome
Click on the menu button > Help and about  > About Google Chrome



or for older versions:
Click on the wrench icon > About Google Chrome



Edge
Click the menu button > Help and Feedback > About Microsoft Edge




Safari
Click the Safari menu > About Safari




Internet Explorer
Click the gear wheel icon  > About Internet Explorer



or for older versions:
Click Help on the menu bar > About Internet Explorer

Opera
Click on the Opera logo in the top left > Go to Help > About Opera