Uppity is a FireFox extension that allows to go up one directory level in the URL of a webpage. This way it allows to do "reversed" browsing: from the ...

Google has updated its URL structure best practices SEO documentation to add new examples, while also making significant changes to the document’s structure.

You can easily keep track of this by examining your URL structure. Each time there is a forward slash after the main domain, this denotes a new directory, adding an additional layer.