The three short horizontal lines on the upper right or left
of browsers and on many apps is commonly called 'the hamburger
menu'.
In Chrome on the
upper-right corner, click on it and under “More Tools” is
'Extensions'. In Firefox it is called 'add-ons'. Click either
and you will see a list of all the extensions or add-ons you
have installed. At the bottom of the list is 'Get more'.
Clicking that will take you to Google Play or Mozilla and show
thousands of free extensions and add-ons you can install.
Incidentally, for
most options, icons, hamburger menus, other menus, Start
Button, or shortcuts in Windows, left click the
mouse to take action, right click the mouse for information.
If you are not sure, right click.