Depends how it was removed
1. If the Add-on was disabled, you can restore it going to Tools -> Add-ons -> Extensions and Clicking "Enable"
2. If the Add-on was un-installed, you can re-install it from addons.mozilla.org
3. If just the icon was removed from the toolbar, you can restore it by clicking the toolbar's 3-line icon Menu -> Customize, and drag the Lightbeam icon back to the main toolbar