SmartBlock stands in for common tracking scripts which are blocked by private browsing mode and strict tracking protection. By doing so it lets pages load more fully, with less breakage, without you having to do anything - all while keeping those tracking scripts blocked.
The difference can be dramatic:
Disabling SmartBlock - Advanced
If you're a website developer or advanced user wishing to disable SmartBlock, you may do so from the Firefox Configuration Editor (about:config page) by setting the preference extensions.webcompat.enable_shims to false. Tracking scripts will then continue to be blocked as before.